I think that if you don't peel the eggplant, the skins have a bitter taste to them, so first of all, peel the eggplant before cooking it.
Cook it with olive oil, onions, bell peppers, and tomato sauce, add Italian Seasoning blend, and I think it should taste fine and not have any aftertaste. You have to add other flavorings to it, as the taste of eggplant is kind of strong. Tomato sauce is what is usually added to it.
You can also mix into it other kinds of vegetables so that the main flavor isn't just the eggplant, like add sliced carrots, sliced yellow crookneck squash.
Also, eggplant does have a very unusual flavor to it, and maybe it is just that you don't like the taste of eggplant very much.