Eggplant has an important place in Provencal cooking, and, as in Italy, tomato commonly assists in its preparation. In the following gratin recipe, the eggplant must be precooked, and in this case, precooking in olive oil lends a particular savor to the final dish. The recipe is from Richard Olney's ''Simple French Food'' (Atheneum, paper $5.95). Eggplant Gratin 1 1/2 pounds eggplant, sliced lengthwise into 1/2-inch slices Olive oil for frying 1 medium onion, finely chopped 1 tablespoon olive oil 1 clove garlic, finely chopped 1 pound tomatoes, peeled, seeded, cut into pieces 1/2 teaspoon sugar Salt, small pinch cayenne 4 ounces ricotta cheese 1 egg Salt and pepper About 1 cup freshly grated Parmesan cheese About 1/2 cup heavy cream Pepper Handful fresh basil leaves
Cook eggplant slices in hot olive oil until golden brown on both sides and tender at all points. Drain on paper towels. For this quantity, fry slices in 3 batches; add additional oil to pan for each, as needed.
Cook onion in oil for 15 minutes until soft and yellowed, but not colored. Add garlic and tomatoes, season, turn up heat, and mix several times until well heated, then turn heat down and simmer for 15 minutes, or until liquid is almost completely reduced. Taste for salt.
Mash white cheese with fork, mixing in egg, first stirring, then beating. Season and stir in enough Parmesan (about 1/2 cup) to bring the mixture to the consistency of a thick paste. Stir in cream until mixture is heavy but of an easily poured, creamy consistency. Taste for salt.
Line bottom of gratin dish or shallow baking dish with half of eggplant slices. Sprinkle with a few grinds of pepper, tiny pieces of basil leaves in an even layer, and some Parmesan cheese. Spoon tomato mixture evenly over surface.
Add remaining eggplant slices and spoon cheese-custard mixture over entire surface. Sprinkle generously with Parmesan and bake in oven heated from 425 to 450 degrees F.
After 10 minutes, turn heat down to 375 degrees F., and bake about 25 minutes , or until surface is puffy and has a rich golden brown. Serves 4.