One of the great dishes of southern France is a thick vegetable soup called pistou. It is a specialty of Nice, and like that city it has close Italian relations. Properly prepared pistou is not only a treat for the palate, but a feast for the eyes that reflects the special color and light of the Cote d'Azur like the shimmering paintings of the impressionist Bonnard.

White beans give the soup a firm base and its crowning glory is the pistou flavoring compounded of fresh basil, garlic, tomato paste, golden olive oil and freshly grated parmesan or gruyere cheese. All of the vegetables should be delicately simmered to retain their freshness and color.

The merciless heat of a hot, humid Washington summer offers few compensations, but one of them is that most of the necessary ingredients for this refreshing soup may be only as far away as your backyard garden or farmer's market.

To make a complete meal of pistou, serve it with loaves of crunch French bread, sweet butter and light wine. Top the repast off with bowls of fresh fruit, plain cookies and coffee.The following recipe is for 10 people and it is an excellent excuse for a party to celebrate the joy of summer. The Soup Serves 10 1/4 cup olive oil 1 cup diced onions 1/2 cup diced celery 1/2 cup finely chopped leeks 2 cups peeled, seeded, chopped tomatoes 3 quarts water 1 cup diced carrots 1 cup diced potatoes 1 cup cooked great northern beans salt and freshly ground pepper to taste 1 cup diced zucchini 1 cup sliced green beans 1 cup fresh peas 1 cup yellow squash 1/2 cup small pasta or broken spaghetti pinch of saffron

In a large soup pot heat oil, add onions and celery. Saute vegetables until softened and slightly browned. Add leeks and tomatoes. Cook until well blended. Add water. Add carrots, potatoes, beans, salt and freshly ground pepper. Simmer gently 10 minutes. Add zucchini, green beans, peas, squash, pasta and saffron. If necessary add boiling water to just cover vegetables. Simmer gently until vegetables are tender. The Pistou 1/2 cup fresh basil leaves 6 cloves garlic 3 tablespoons tomato paste 1/2 cup freshly grated parmesan or gruyere cheese 1/2 cup olive oil

With either a mortar and pestle or wooden spoon and heavy bowl, mash basil and garlic into a paste. Add the tomato paste and cheese. Blend thoroughly. Gradually beat in the olive oil a tablespoon at a time to make a thick sauce.

To serve: Place the pistou in a soup tureen. Add about a cup of the hot soup. Stir thoroughly. Gradually add the rest of the soup to the tureen. Garnish with chopped parsley.

Note: Hudson Brothers, Inc., 3206 Grace St. NW, 337-8585, will have bunches of fresh basil until the first frost. The Georgetown Market Cheese Stand, also at 3206 Grace St., 337-8737, will grate parmesan cheese to order.