In stews and purees, eggplant's bitterness usually isn't a problem. But using a cooking method like frying or grilling concentrates its bitter character. Here are two methods for bringing the best out of eggplant:
SALTING: Slice the eggplant, then sprinkle salt (coarse salt is best because less is absorbed) onto one side. Leave it for 30 minutes, to allow the solanine (a chemical found in the flesh) to leach out, and brush off with a damp cloth.
CHILLING: Arrange eggplant slices on a plate and put it in the freezer for about 4 hours. When the slices thaw, press out a lot of the water with the palm of your hand, releasing most of the bitterness. The eggplant will fry as if it had been salted, though the freezing will make the flesh fall apart a bit more.