The skillful use of onions, some say, is the sign of a good cook. It is certainly true that all good cooks have onions handy in the kitchen.

Since earliest written history, the pungent and assertive flavor of the onion family has enriched nearly all cuisines. The first onions were probably from Southeast Asia or the area around the Mediterranean. History records that desert travelers and soldiers on the march used onions as thirst quenchers. Onions that have been harvested and dried for a time (as they are before marketing today) keep well if they are kept cool and dry. So we can imagine the ancient men shading onions in baskets or bags of some kind for their travels in arid land.

Today we store our onions in a cool, dry place, as dampness and excessive heat cause spoiling and sprouting. A well-ventilated onion basket is essential in many kitchens.

While the onion family also includes leeks, chives, shallots and garlic, in the United States when we think of onions we most often think of the yellow or white globe onion. Indeed, this onion accounts for 75 percent of the onions cultivated here. So many sizes of onions are available, it can be perplexing, but in general terms, onions are divided by size and flavor into various types suitable for different purposes. The smallest, called the button or pearl onion, is the pickling onion. It can be yellow or white. Next comes the boiling size, about 1 1/4 to 1 1/2 inches in diameter. Larger onions with increasingly strong flavor are best for chopping and grating. However, the largest onions, the spanish or bermuda onions, are characterized by their mild, slightly sweet and delicate flavor. They can be white, yellow or red. These spanish onions are superb when peeled and sliced crosswise. They are perfect with a hamburger or in a salad, or finely chopped to accompany caviar. There are those onion lovers who say the onion is at its best raw, and certainly these mild varieties give credence to their verdict. Also good raw are slim, green onions or scallions. They are excellent served with salt alone, or cut into salads, as well as braised and served like asparagus -- hot or cold.

It is in cooking, however, that the onion is most versatile. For a long time in this country onion was considered merely a seasoning rather than as a vegetable on its own. We are much more creative today, however, and onions are served stuffed (as a main course or accompaniment) and cooked with cream, braised, boiled, baked, deep-fried and, of course, in sauces, soups and stews. Thrifty French cooks have even taught us a trick with onion peels: papery outer skins of onions -- except the white varieties -- have the ability to darken any liquid to which they are added; so French cooks sometimes add an unpeeled (but cleaned) onion to a stew or soup or stock to darken it to a richer color. At Easter, the French also color eggs yellow by hard-cooking them with some of the papery, outer skins of the onion. If each egg is wrapped in these papery peelings, and then cheesecloth is secured around each before boiling, the result will be a marbled effect when the eggs are done. When they are cool, rub the eggs with a little bit of oil or butter for a soft glow much like alabaster.

Onion soup can be made with any type of onion. However, really delicate and superbly tasty and fragrant French onion soup should ideally be made with spanish or bermuda onions. These large onions, while mild and characteristically moist, have a higher sugar content than other onions. In the long saute'ing that is essential to good onion soup, the sliced onions release their natural sugars, which caramelize and impart a distinctive flavor and rich mahogany color to the soup. In other cases, where long simmering is necessary and the onion must not lose its flavor, but impart it to a blend of ingredients, a stronger onion would be better. Spanish onions or even large globe onions are good for stuffing and baking.

Here are a couple of hints for onion fanciers: When you have a big batch of those white onions to peel, drop them, unpeeled, into boiling water. Count to 10 slowly, and immediately cool the onions with cold water. Drain them, and with a little help from a small paring knife, the outer skin will peel off easily, leaving the onions unscarred. (These onions are good not only in traditional cream sauce, but braised in chicken stock, or added to stew at the last minute. Just do not boil them to disintegration.) You've probably heard this, but it bears repeating: hold onions under cold water when peeling them. This prevents the volatile oils from rising and causing tears. And after all the peeling is done, make a paste of baking soda and water, or salt and water and rub this all over your hands; rinse thoroughly and the onion odor will disappear from your hands.

Leeks, which look like big, fat scallions, are mild-flavored and delicious members of the onion family. They are much overlooked in our cuisine, and it is a pity. The finest continental chefs would be at a loss without an abundant supply of leeks for their stocks and soups. While leeks are grown primarily for their roots, many cooks use the tender, young green part. Leeks are served cooked, and in fact, they may be prepared in any way that is suitable for asparagus. Thus they are often called "poor man's asparagus," though where leeks are little in demand and therefore expensive, they can outprice asparagus.

Leeks, like their cousins the onions, have been around since ancient times. The leek, in fact, is the national emblem of Wales. The legend goes that when Cadwalader, the Welsh leader, was about to face Edwin, the King of Northumbria, in battle, he instructed his soldiers to stick leeks in their helmets so he could distinguish them from the enemy. The Welsh were victorious, and the leek was adopted as the national emblem.

When buying leeks, look for bunches in which the root sections are no larger than 1 1/2 inches in diameter; larger ones tend to be pulpy. To prepare leeks for cooking, strip off the outer leaves, cut off the root, and snip the green part down to where it is tender. Some cooks discard the green part altogether, considering it too coarse, but if the greens are tender, they are good when cooked. Generally speaking, it is necessary to cut the green part down to five or six inches. Leeks are always full of grit and dirt, and must be washed thoroughly. Be very careful to rinse out all pockets of grit. Then the leeks may be cooked quickly in boiling salted water . . . no more than 15 or 20 minutes. They are good served hot or cold. Try boiling them with tiny small onions, or serving them hot with your favorite mushroom sauce, or butter and lemon juice, or drain and chill them and serve them with a vinaigrette. Leeks are familiar as an essential ingredient in vichyssoise, and just as important but less familiar in Scottish cock-a-leekie soup. This is a simple soup, a recipe that hasn't changed in centuries. It consists of a tough old fowl (some say the tougher the better) combined with layers of finely sliced leeks and a few quarts of water, then simmered for hours. Sometimes a handful of barley is added to the soup, but it isn't essential. A food historian tells us that "Cock-a-leekie is ready when the fowl is rags, the leeks are pulp, and the broth is lovely."

A good cook is defined as one who "knows his onions." Here are some recipes for onions and leeks to prove it.

STUFFED ONIONS (6 servings) 6 spanish onions Cold cooked veal or chicken 1/4 teaspoon salt 1/2 teaspoon pepper 1 teaspoon chopped parsley 1/4 cup soft bread crumbs 1/4 cup butter 1 tablespoon chopped cashew nuts Buttered cracker crumbs

Peel onions. Gently parboil (in water to cover) or steam whole onions until they are tender, but not mushy. (This may take as long as an hour if the onions are quite large.) Remove from fire and cut a circular piece from the top of each onion. Then scoop out the insides to form cups. Chop finely the onion pulp that has been taken out. To this add an equal amount of finely chopped cooked veal or chicken. Add salt, pepper, parsley, bread crumbs and butter, and mix thoroughly. Put a spoonful of the mixture into each onion. Then add cashew nuts to the mixture, stir, and finish filling the onions. Place onions in a baking dish just large enough to hold them, and bake slowly at 325 to 350 degrees for 40 to 60 minutes, depending on size of onions. Baste three or four times with butter melted in water. Fifteen minutes before onions are to be served, sprinkle top of each onion with buttered cracker crumbs and return to oven to brown. (These stuffed onions are good served with a thin cream sauce -- see recipe below.)

THIN CREAM SAUCE (Makes 1 cup) 1 tablespoon butter 1 tablespoon flour 1 cup light cream Salt and white pepper to taste

In a saucepan melt butter and stir in flour to make a paste. Cook, stirring, for 2 or 3 minutes but do not allow to brown. Add light cream, stirring constantly over low heat until slightly thickened. Add salt and white pepper to taste and serve at once.

BAKED ONIONS IN FOIL (1 onion per serving) Bermuda onions (1 per serving) 1 tablespoon butter for each onion 2 dashes worcestershire sauce for each onion Seasoned salt

Peel onions and make 1 or 2 slits in top of each onion. Force butter into slits. Place each onion on a square of heavy duty aluminum foil. Dash with worcestershire sauce, sprinkle with seasoned salt. Fold foil up and seal. Bake in 375-degree oven about 20 minutes, or until done. Very good with steak.

CHEESE AND ONION PIE IN CRUMB CRUST (4 to 6 servings) 30 saltine crackers, rolled fine 1/4 cup butter, melted 2 1/2 cups thinly sliced onions 1 tablespoon butter or margarine 1/2 pound grated sharp cheddar cheese 3 eggs, slightly beaten 1 1/2 cups milk Salt and pepper to taste

Combine cracker crumbs and butter and press into 9-inch pie plate. Saute' onions in butter just until limp. Place onions in the pie plate, cover with grated cheese. Combine eggs, milk and seasonings and pour over onions. Bake in preheated 325-degree oven about 50 minutes, or until firm. Cut in wedges and serve.

ONION SOUP (8 servings) 1 1/2 to 2 pounds bermuda onions, thinly sliced 1 clove garlic, minced 3 tablespoons butter 1 tablespoon olive oil 3 tablespoons flour 6 cups beef broth 1 cup red wine 1/2 teaspoon sage 1 bay leaf Extra pieces swiss cheese Thick slices french bread (or toasted thin slices) 1 cup swiss cheese, grated

Cook onions and garlic in butter and oil, covered, for 15 minutes over moderate heat. Raise heat and remove cover. Cook 15 minutes until onions are golden brown. Stir in flour and cook 2 minutes. Add more butter if necessary to form a paste. Remove from heat. Stir in broth, wine and seasonings. Simmer 15 to 20 minutes. Place in casserole (or individual ones). (May be made ahead and refrigerated at this point.) Drop extra cheese pieces into soup. Place slice of bread on top and cover with grated cheese. Bake at 250 degrees 15 to 20 minutes, or until bubbly. Serve immediately.

BRAISED LEEKS (4 servings) 1 large bunch leeks 1 medium carrot 4 sprigs fresh parsley 1 large bay leaf 1/2 cup minced celery 1/2 teaspoon salt Pinch of thyme 1/8 teaspoon pepper 2 whole cloves 1 1/2 cups stock or beef bouillon

Wash leeks thoroughly and trim the green tops to where they are tender. Peel the outside skin, and remove the root. Scrape the carrot and cut in thin slices. Place leeks in the bottom of a baking dish and cover them with vegetables, spices and seasonings. Add the stock or bouillon, cover tightly and bake in moderate oven (350 to 375 degrees) about 45 minutes. Drain and serve hot with the following sauce:

SOUR CREAM SAUCE (4 servings) 2 eggs 3/4 cup sour cream 1/2 teaspoon lemon juice 1/2 teaspoon salt 1/2 teaspoon paprika

Beat the eggs slightly, add the cream and seasonings and cook in double boiler over hot water until sauce thickens. (This sauce is also very good with cauliflower, asparagus or broccoli.)

LEEK SOUP WELSH STYLE (6 servings) 4 large leeks 4 medium potatoes 6 cups water Pinch of salt 1 medium onion 2 teaspoons butter or margarine 2 tablespoons flour Pinch of salt 2 egg yolks

Wash and trim leeks. Cut them in half lengthwise, then crosswise into thin slices. Peel the potatoes and dice them. Simmer potatoes with the water and salt until tender. Peel and chop onion finely. Melt the butter in a skillet and cook the leeks and onion in the butter until transparent, but do not brown. Stir in the flour and cook a few minutes to blend in the flour. Moisten with a little of the potato water and stir to make a smooth paste, then combine leeks with potatoes and continue simmering until they are very soft. Then pure'e the vegetables in an electric blender (or try forcing them through a sieve) and return pure'e to saucepan to reheat. Beat the egg yolks and place in the bottom of a soup tureen. Stirring vigorously, pour the soup on top of the eggs and stir in 1/2 cup cream, if desired. Serve immediately.

TARTE A L'OIGNON (Belgian Onion Pie) (4 to 6 servings) 1/2 pound butter 3 large yellow onions, sliced into thin half-moons 3 tablespoons flour 1/2 cup milk, warmed 1/4 cup cream, warmed Pinch of nutmeg 2 eggs at room temperature Salt Hot pepper sauce to taste 1/2 cup freshly grated parmesan or romano cheese 8-inch baked pie shell

Melt butter in skillet and saute' onions until transparent. Cover and cook onions until they are limp and just start to color. Continue to cook, stirring, until the onions are very slightly but evenly browned. Sift the flour over the onions, stirring constantly to prevent lumps. Gradually add the warmed milk and cream. Add pinch of nutmeg and stir to a smooth paste. The mixture should be the consistency of mashed potatoes. Remove from heat. Beat eggs thoroughly and blend in. Add the cheese, salt and hot pepper sauce to taste. Mix well. Pour the mixture into the prebaked pie shell and bake in 400-degree oven 30 to 40 minutes, until the center is set and the top is a delicate straw color. (The filling may be made ahead, but do not put it into the pie shell until baking time.)

SWISS LETTUCE AND LEEK SALAD (4 servings) 1 head lettuce 3 leeks 3 slices bacon 2 to 3 tablespoons vinegar 1/4 teaspoon salt

Shred the lettuce, chop the leeks and mix together. Fry the bacon and crumble it over the salad. Add the vinegar and salt to the bacon drippings, and pour warm over the salad. Serve at once.

MARINADE-DRESSING FOR COOKED LEEKS (Enough for 2 pounds leeks -- about 6 servings) 1/3 cup fresh lemon juice 1/4 cup olive oil 2 tablespoons snipped chives 2 teaspoons capers 1 1/2 teaspoons sweet basil 1 teaspoon cracked pepper 1/2 teaspoon anchovy paste

Mix all ingredients and pour over cooked leeks. Marinate overnight, serve on crisp salad greens. (Also good for cooked asparagus spears).

FRENCH-FRIED ONIONS (4 servings) 4 medium-large yellow onions Milk Flour Oil for frying (2 or more cups) Salt and pepper

Peel and cut onions into 1/2-inch slices and separate into rings. Dip each ring in milk, then in flour and fry in hot (380-degree) oil until brown and crisp. Drain and serve. Add salt and pepper to taste.