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.