When I first introduced leeks to a cooking class eight years ago, barely half the students had seen them and no one [WORD ILLEGIBLE] them in cooking. In the recent years, however, with leeks commonly found at supermarkets instead of just specialty shops, this vegetable is gaining in use and popularity.

Botanically the leek may be related to garlic rather than onion. The blossom of a mature leek, in early spring, resembles that of garlic, and its starburst bloom, sometimes three inches across, is astonishingly attractive and can be dried and used in arrangements.

Although the first use of leeks is obscure there are early references to their qualities in Egyptian and Roman history. In modern cooking they are the undisputed kings of the stockpot. To leave leeks out of the bouillabaisse is almost as bad as leaving out the fish. Their pungency while cooking belies a gentle flavor. When served as a separate vegetable leeks vie with asparagus in elegance, exceed them in availability and surpass them, unfortunately, in cost.

The leek comes complete to the marketplace - root, bulb, stalk, leaves and dirt from the garden in which it grew. This necessritates careful cleaning. Trim the root end carefully so the layers of the stalk remain attached at the bottom. This is especially important if you are braising them and want them to stay together. If you are using the leeks for making stock, leave two or three inches of the green part on the plant, otherwise trim down to the white stalk.

At this junction of white and green, where the stalk has pushed out of the soil, you will find a quantity of grit. Cut the leek in half lengthwise and hold it under cold running water, rifling the layers with your thumb to allow the water to wash out the sand. Inspect and repeat until clean. Drain on toweling and proceed to the recipe. Among the following selections are a delicate, flaky leek tart, a classic leek and potato soup, and two proparations for leeks as hors d'oeuvre or vegetable. LEEK TART 2 1/2 cups all-purposr flour 1 teaspoon salt 2 sticks (1/2 pound) sweet butter, very cold 6 to 8 tablespoons ice water 8 to 10 large leeks, trimmed to the white part, washed and diced 1 stick (1/4 pound) sweet butter Salt and fresh ground pepper 2 egg yolks 1/4 to 1/3 cup heavy cream

Mix the flour and salt in a bowl, put in the butter and mix it in with a pastry blender until a coarse crumbly mixture is blender until a coarse crumbly mixture is achieved. Add just enough water to hold the pastry together, roll into a ball and chill for several hours before using. Divide the ball of pastry into two uneven amounts, so that the bottom crust is a little thicker than the top crust. Roll out the bottoom and line a 10-or 11-inch tart pan (preferably with removable bottom) with it, allowing at least a one-inch overlap.

Bring three or four quarts of salted water to a boil, throw in the diced leeks, pull off the heat and let sit for five minutes. Drain well. Melt the butter in a saute pan and cook the leeks very slowly in the butter, without browning, until most of the butter has been absorbed. Set aside to cool. Salt and pepper to taste. Fill the crust.

Roll out the top crust to the exact diameter of the pan uou are using. Mix the egg yolk and heavy cream together and brush a little of this mixture around the edges of the top crust. Bring the overlap down and seal the pastry. Cut a small hole in the center and brush the crust lightly with the egg yolk and cream mixture. Bake at 425 degrees, in the center of the oven, for about 35 minutes. Immediately upon removal from the oven pour the remainder of the egg yolk and cream mixture carefully through the hole in the top pastry, tilting rapidly so that it will spread throughout. Serve while still hot in small wedges. LEEK AND POTATO SOUP 4 large leeks deced (white part only) 1 medium yellow onion, diced 1 or 2 tablespoons sweet butter 4 medium Idaho potatoes, pared and diced (1/2 inch dice) 3 cups chicken stock Salt and fresh ground white pepper 3 cups hot milk

Saute leeks and onion slowly in one tablespoon of the butter, in a heavy pot. Do not let them brown. When they have wilted somewhat, add stock, potatoes and one tablespoon salt. Bring to a boil, reduce the heat to simmer, cover, and cook for 20 to 25 minutes, or until the potato cubes are just tender. Put mixture through the medium disc of a food mill. Return the puree to the pan, reheat and add hot milk and an additional one tablespoon butter if desired. Correct seasoning and serve GREEK-STYLE LEEKS 4 tablespoons lemon juice 6 tablespoons olive oil or salad oil 1 cup pous 2 tablespoons chicken stock 1 teaspoon crushed fennel seed 1 clove chopped garlic 1 to 2 teaspoons crushed coriander seeds A pinch or two of dried thyme, or a small bunch of fresh thyme A small bay leaf, broken into pieces Fresh ground pepper, salt to taste 8 leeks, white part only, split, cleaned, still attached at the root end

Combine all the above ingredients, except the leeks, and bring to a boil, preferably in a skillet. Simmer for 15 to 20 minutes. Add the prepared leeks (in one layer, if possible) and continue to cook very slowly for 20 to 30 minutes, depending upon the youth and tenderness of the leeks. They will become quite limp and should be removed carefully with a spatula. Set them on a serving plate or platter and pour some of the liquid over them. Let cool to room temperature before serving. A bit of chopped fresh parsley or a garnish of watercress would be appropriate. BRAISED LEEKS WITH RAVIGOTE SAUCE 2 cups chicken stock 6 to 8 leeks, left whole if small, split if large, well washed 3 tablespoons wine vinegar 1 tablespoon good Dijon mustard 6 tablespoons olive oil 3 hard-cooked egg yolks, sieved A branch of fresh tarragon, chopped, or a large pinch of dried tarragon Small handful of chooped fresh parsley Salt and fresh ground pepper

Bring the chicken stock to a boil in a flat pan (skillet). Add the leeks and simmer them until tender, 20 to 30 minutes. Remove them carefully and drain them. Cool to room temperature before serving.

Arrange the leeks on individual serving plates. Cover with the sauce ravigote, made as follows: Mix the vinegar, mustard and oil together with a fork or whisk. Add this mixture to the sieved egg yolks, a little bit at a time, mixing thoroughly until smooth and slightly thickened. Season with tarragon, parsley, salt and fresh ground pepper.

Do not mix this sauce too soon before using as it has a tendency to separate. It is a strong, rich, spicy sauce excellent on all manner of cooked cooled vegetables, or poached fish. Use sparingly.