AS OUR Washington summer slouches wearily toward its conclusion, the gardening cook -- or the coking gardener -- faces the turn dilemmas of a garden that keeps churning out vegetables even after alternatives for their use decrease with each tomato-squash casserole.

The following pureed vegetable soups, while not providing any answers for the major policy questions of life, do hack away a bit at the sometimes knottier question of what to have for dinner. They can be served icy cold when it's steamy outside, or comfortably hot on the first day that early sunset forewarns of winter's approach.

Once you get the hang of making this kind of soup, you'll be prepared for all kinds of culinary emergencies because they can be made with nearly any vegetable, and the technique for all is the same.

The idea is to start with a little onion or leek softened in a bit of butter, then to add the vegetable in question, enough water or stock to cover, plus a thickening agent such as rice or potato, and seasonings. Some kind of enrichment -- cream, yogurt or sour cream -- can be added at the end, along with fresh herbs.

The recipes given here take advantage of vegetables that are plentiful now, but the variations on the main theme are nearly endless. In the winter you could use potato or turnips, broccoli or pumpkin. When spring comes around again, try fresh peas or asparagus. One of the beauties of these soups is that you can use the stems of vegetables such as broccoli and asparagus, relieving that vestigial guilt about discarding edible waste.

Here are the basic components:

* Leek or onion. It doesn't matter much which is used. Onions tend to be sweet when cooked, though some cooks think leeks have a more interesting flavor. But don't run out and buy leeks if all you have is onions and half an hour until dinner time. Feel free to use both in greater or lesser quantities according to your taste. Note that these recipes call for "sweating" the onions or leeks for a few minutes in butter, a step that concentrates flavor but can be eliminated in a pinch.

* The vegetables. For once, you get to do whatever strikes your fancy. Keep in mind the color of the finished product and avoid using red and green together unless you love olive drab. How you plan to pure'e the soup (see below) determines whether you must peel and/or seed vegetables such as tomatoes and cucumbers.

* The starch. Using rice or potato avoids the trouble of making a white sauce or an egg yolk-cream liaison to thicken the soup. Some vegetables -- especially acid ones such as tomato -- go better with rice, others potato, but either will do. Macaroni or other pasta also works. If after puree'ing you find the soup too thin, you can always simmer it a while (uncovered) to thicken, or add another potato or tablespoon of rice, simmer for 20 minutes or so and puree again. The extra reduction will only help concentrate flavors.

* The liquid. Chicken stock adds flavor and richness and is especially good if you plan to serve hot soup. Rich, homemade chicken stock, however, becomes gelatinous as it cools and makes cold soups too thick and stiff. A combination of stock and water, or water alone, seems to work best for cold soups. Canned chicken stock, or that made from bouillon cubes, will not become gelatinous. On the other hand, both of these are far removed from the real thing.

This is a good time to use that extra broth from recipes you've made which said, "save the broth for soups." Meat broths most commonly add flavor to soups, but vegetable broths, which contain some vitamins, are also appropriate. Use strong broths for substantial soups, however, being careful not to mix broth from cooking cauliflower as a base for delicate cream of cucumber soup.

Some vegetables such as cucumbers and tomatoes contain lots of water, and the water content of vegetables generally varies according to how rainy the growing season was. Thus it's better to add the minimum amount of liquid first and adjust later with water, stock or milk.

* The seasonings. When a soup needs something extra, remember nutmeg, ground red pepper, celery seed, dry or prepared mustard, and -- best of all -- fresh herbs, which add not only flavor but also color. Keep in mind that cold soups require more seasoning than hot.

* The pureeing. Using a food mill eliminates the necessity for peeling vegetables such as tomatoes and allows the cook to squeeze the flavor out of ingredients while leaving their tough or stringy qualities behind. If you use a blender or food processor for pure'eing, you might want to strain the soup afterward to remove errant strings.

* The enrichment. Use milk, cream, creme fra iche', yogurt, sour cream or butter. (Use butter only if the soup will be served hot.) These are added after pure'eing, when you have tasted your soup and decided how you feel about it. Some soups, such as cold cucumber, seem to need heavy cream, which adds richness without masking the suave flavor of the cucumbers.

* Garnishing. When food looks pretty, people assume you've slaved over it. Paper-thin slices of tomato or cucumber floating on the surface of these soups will make your guests feel pampered without wearing you out in the process. Present the soup topped with chopped herbs and/or sour cream. A rosette of whipped cream piped on top of the soup through a pastry bag is simple elegance perfect for company.

If you plan to serve these soups cold, make sure they are very well chilled, at least three hours. In fact, they are even better if left refrigerated overnight.

If you plan to serve them hot, they can be made ahead and reheated. Remember that yogurt and sour cream tend to curdle if boiled, so reheat carefully or add enrichment just before serving and heat through.

All these soups will keep for several days in the refrigerator. CUCUMBER SOUP (6 servings)

This is an elegant soup when served cold. To serve it hot, add another potato and, if you like it, a handful of watercress leaves. 1 tablespoon butter 2 cups leeks, sliced (use white and tender green parts only) 1 medium baking potato peeled and sliced 4 cups peeled, seeded cucumber cut into 1-inch slices 2 cups water 1 cup chicken stock Salt, pepper and grated nutmeg to taste 1/2 to 1 cup heavy cream

Chopped fresh chervil, chives, or parsley, or paper-thin slices unpeeled cucumber for garnish

Melt butter in a large heavy-bottomed saucepan. Add the leeks and cover. Allow them to cook ("sweat") over low heat for about 10 minutes. Do not let them brown. Add potato, cucumber, water, stock and seasonings, and let simmer over medium-low heat until the cucumber is soft, about 30 to 40 minutes. Pure'e and add cream. Let cool, then chill well. Before serving adjust seasoning and sprinkle with herbs, or garnish with paper-thin slices of unpeeled cucumber that have been scalloped by running the tines of a fork down its length. CURRIED TOMATO SOUP (6 servings)

The flavoring in this energetic soup isn't enough to scream "curry!" but it does cut the acidity of the tomatoes. A little sugar can be used, with or without the curry powder, to achieve the same effect. 1 tablespoon butter 1 1/2 cups sliced leeks, white and tender green part only 6 cups chopped tomato 3 tablespoons uncooked rice 1 cup chicken stock 2 cups water 1 teaspoon curry powder, or to taste 1 bay leaf 1 celery stalk Few parsley sprigs Salt and ground red pepper to taste 1 to 3 tablespoons sugar or to taste (optional) 1/2 to 1 cup heavy cream, yogurt, sour cream or creme fraiche' Fresh herbs or paper thin slices of tomato or both

Melt butter in a large heavy-bottomed saucepan. Add the leeks and let soften, covered, over low heat for about 10 minutes. Do not let the leeks brown. If you plan to pure'e the soup in a food mill, you can leave the tomatoes unpeeled, otherwise they must be peeled. Core the tomatoes, squeeze to remove juice and seeds, and cut into 1-inch cubes. Add the chopped tomatoes, rice, stock, water and the rest of the ingredients except sugar, cream and garnish to the leeks. Bring to a boil and let simmer until the tomatoes are soft, about half an hour. If you are using a blender or food processor, remove the celery stalk, parsley stems and bay leaf, then pure'e.

Add cream, yogurt or creme fra iche' to taste and adjust seasonings, adding sugar if necessary. Serve hot, or chill thoroughly. Taste again before serving for seasoning and garnish with fresh herbs, paper-thin slices of tomato, or dollops of whipped or sour cream. CARROT-APPLE SOUP (6 servings) 1 tablespoon butter 1 cup sliced leeks, tender green and white parts only 4 cups carrots, scraped and cut into 1-inch slices (about 10 carrots) 1 medium potato, peeled and sliced 1 tart apple, peeled and sliced 4 cups water Few sprigs fresh thyme, or a pinch dried thyme Salt and pepper to taste 1/2 to 1 cup heavy cream, yogurt, sour cream or creme fraiche' Fresh herbs or finely-julienned strips of carrot for garnish

Melt butter in a large heavy-bottomed saucepan. Add leeks and let soften, covered, over low heat for about 10 minutes. Do not let them brown. Add the rest of the ingredients except cream or yogurt and garnishes, bring to a boil and let simmer for about 40 minutes, until the carrots are soft. Puree, removing fresh thyme stems first if you are using a food processor or blender.

Add cream or yogurt, adjust seasonings, and serve hot or cold. For garnish, sprinkle with fresh thyme leaves or other fresh herbs, or with julienne strips of carrot softened for a minute in boiling water. ZUCCHINI SOUP (6 servings)

Even monster zucchini can be used in this soup, and the mustard makes it peppier. 1 tablespoon butter 2 cups onion, peeled and sliced 3 tablespoons uncooked rice 4 cups zucchini scrubbed but not peeled, cut into 1-inch pieces 2 cloves garlic, peeled and sliced 2 cups water Prepared or dry mustard to taste Salt and cayenne pepper to taste Water, chicken stock or milk for thinning, if necessary 1/2 to 1 cup yogurt, sour cream, cream or creme fraiche' Fresh herbs for garnish

Over low heat, allow onion to soften in butter for about 10 minutes in a covered, heavy-bottomed saucepan. Do not let onion brown. Add rice, zucchini, garlic, water and seasonings and bring to a boil over medium heat. Let simmer for 30 to 40 minutes over medium-low heat, or until zucchini softens.

Puree. Zucchini varies in moisture content, and the puree may be too thick. If so, add water, chicken stock or milk, plus the enriching cream or yogurt. Adjust seasoning and serve very hot or very cold, sprinkled with fresh herbs (try dill or parsley).