One of the best but least known French fish dishes from Provence is a thick, creamy soup-stew called most popular sauces, aioli , a pungent garlic mayonnaise that is stirred into the stock in which the fish was cooked to make a lovely, smooth yellow sauce.

Although sometimes described as a variation of bouillabaisse, bourride is not easy to define as there are a number of ways to make it. Actually it is an aromatic main dish that is served in a wide soup plate with the soup broth ladled over toasted French bread and the fish and garnished with aioli sauce. In Provence it is a specialty.

Unlike bouillabaisse, bourride is not difficult to prepare away from the Mediterranean as it does not require particular species of local fish and does not generally include shellfish. Some recipes call for conger eel and perhaps mussels, but generally only white fish - two or three different firm-fleshed types such as sea bass, sole, mullet, flounder or bream are used. Haddock, scrod, cod, perch, rockfish, whitefish and pollack all are suitable. The fish should be as fresh as possible and purchased with the trimmings which are necessary for the stock or court bouillon.

Seasonings for bourride also vary. Some versions include saffron but more typical are thyme, bay leaf, fennel and orange peel. Purists condemn the use of tomatoes or tomato puree in bourride.

In Marseilles restaurants, the spicy red rouille traditionally served with bouillabaisse, and aioli are served with bourride. Only the latter is essential, however, as the pleasure of the dish heightened by the aioli's garlic and olive oil flavor.

Sometimes called the "butter of Provence," aioli is also served with boiled codfish and raw and cooked vegetables, especially at traditional weekly Friday luncheons and for Christmas Eve suppers in Provence. Originally made with olive oil an garlic and thickened with breadcrumbs, the modern version is a thick, smooth, golden mayonnaise made pungent with at least one garlic clove per person. Provencal cooks painstakingly pound the garlic in a mortar with a pestle, and then carefully add the egg yolks, salt, olive oil and lemon juice. Sometimes breadcrumbs and wine vinegar or a mashed boiled potato are used as thickeners. Some cooks also add a little cold water.

Making a good aioli , like a good mayonnaise, can be tricky and requires attention, the best olive oil and garlic that is free of blemishes anf of any green sprouts. Most of the preliminaries for a bourride can be done before the final cooking, which takes about 15 minutes. Since the dish should be served exactly when finished it does require last-minute attention. Serve with hot boiled potatoes and green salad, if desired, and accompany with a robust, dry white wine. A good dessert would be a fruit-filled pastry or fresh fruit. The following version is similar to a bourride I enjoyed in a colorful seaside restaurant in Saint Tropez. PROVENCAL BOURRIDE

(Serves 6) 3 1&2 to 4 pounds mixed firm-fleshed white fish (sea bass, rock, haddock, flounder, cod) 6 cups water 1&2 cup dry white wine 2 tablespoons wine vinegar, preferably white 2 medium onions, peeled and sliced thinly 2 bay leaves 1 teaspoon fennel seeds 1 teaspoon dried thyme 2 small strips orange peel, white rest removed 2 teaspoons salt 3 egg yolks Aioli (recipe below) Salt, white pepper to taste 1 tablespoon fresh lemon juice (optional) 12 pieces toasted French bread

When purchasing the fish, if you have it filleted, ask that heads, hones and trimmings be wrapped as well.

Put these heads, bones and trimmings in a large kettle. Add water, wine, vinegar, onions, bay leaves, fennel, thyme, orange peel and salt. Bring to a boil. Lower heat and cook slowly, partially covered, for 30 minutes. Skim off scum occasionally. When cooked, strain broth into a bowl, pressing ingredients with a wooden spoon to extract all juices. Wash kettle and return strained liquid to it.

About 15 minutes before serving, bring the broth to a boil and add the fish, cut in serving pieces. Lower heat and simmer, covered, for 5 to 8 minutes, until firm to the touch. Do not overcook. Carefully remove the fish with a slotted spoon to a warm platter and cover to keep the fish warm.

Then quickly beat the three egg yolks, one at a time, with a wire whisk into 1 cup of aioli sauce in a saucepan. Slowly stir in 1 cup of the hot fish broth and cook over low heat, stirring constantly until the broth is thick enough to coat the whisk lightly.Season with salt and pepper and add the lemon juice if desired.

To serve, pour the broth into a tureen or large bowl and bring to the table with the warm toast, platter of warm fish and reserved aioli. Each person puts two slices of toast in the bottom of an individual, wide soup plate and tops them with one or two pieces of fish and some hot broth. The aioli is passed separately. GARLIC MAYONNAISE

(Aioli) 6 medium garlic cloves, peeled and crushed *3 egg yolks at room temperature 1&4 to 1&2 teaspoon salt White pepper to taste (optional) 1 1&2 cups olive oil at room temperature 2 to 3 tablespoons fresh lemon juice.

Pound garlic in a mortar or bowl with a pestle or wooden spoon. Add egg yolks, one at a time, and pound together until well blended and thick. Season with salt and pepper. Then begin adding oil, drop by drop, beating constantly with a wire whisk until mixture begins to thicken.

Add half remaining oil in a steady stream, beating constantly. Then add lemon juice and remaining oil, still beating steadily, until thickened and smooth. Put 1 cup sauce in a small bowl to be added to the soup and put remaining sauce in another small bowl or saucepan to be passed with soup at the table.

Note: The aioli may be prepared beforehand or may be made while the soup stock is cooking. It may also be prepared in an electric blender if the oil is added very slowly.