Eggplant gratin

2 lb Japanese or round eggplants
           Oil for frying
      3 tb Virgin olive oil
      1    Red onion; finely chopped
      1    garlic clove; finely chopped
    1/2 ts Herbes de Provence
  2 1/2 lb Ripe tomatoes
           - peeled, seeded and chopped
           Salt
           Pepper, freshly milled
           Sugar, if needed
    1/8 ts Saffron threads
      2    Eggs
      1 c  Ricotta cheese
    1/3 c  Milk or whipping cream
    1/2 c  Grated Parmesan cheese
     10 lg basil leaves; cut or torn
           - into small pieces
      3 oz Gruyere cheese
           - shredded or thinly sliced
 
  SLICE EGGPLANTS diagonally about 1/2-inch thick. Discard pieces that are
  all skin on one side. If using round eggplants, halve them lengthwise, then
  cut into 1/2 or 1/4 rounds, about 1/2-inch thick. Generously cover bottom
  of wide skillet with oil. When hot, add eggplant in single layer and fry
  until golden on both sides and flesh is tender. Drain on paper towels. Add
  more oil, if needed, and fry rest of eggplant. Make sure oil is hot each
  time you add a new batch. FOR SAUCE: Warm olive oil in skillet and add
  onion, garlic and herbs. Stir to coat onion with oil. Cook gently until
  onions soften, 12 to 15 minutes. Add chopped tomatoes, salt lightly and
  raise heat. Cook, stirring occasionally, until juice has evaporated and
  sauce is thick, about 15 minutes. Season to taste with salt and pepper. If
  tomatoes are on the acid side, add sugar to taste to correct balance. FOR
  CUSTARD: Cover saffron threads with a few tablespoons hot water and let
  stand a few minutes. In bowl, beat eggs and stir in ricotta cheese, milk
  and Parmesan cheese. Season with salt and pepper, then stir in saffron with
  liquid. FOR GRATIN: Spread a little tomato sauce over bottom of baking
  dish, then arrange a layer of eggplant slices, slightly overlapping. Season
  with salt and pepper, scatter half the basil, add Gruyere. Make another
  basil-eggplant layer. Cover with rest of the tomato sauce. Pour custard
  over the top. Preheat oven to 350F and bake 40 minutes or until custard
  sets and is a delicate golden brown. Remove from oven and allow to rest a
  few minutes before serving.