Preheat oven to 375°. Peel eggplant, if desired, and cut into 1/4-inch-thick slices. Arrange half of the eggplant slices in a lightly greased 11-by-7-inch baking dish that's 1 1/2 inches deep. Layer half of the Parmesan cheese over eggplant, followed by 3/4 cup mozzarella and half of the spaghetti sauce. Repeat.
Cover dish and bake for 40 minutes or until eggplant is tender. Uncover, top with remaining 1/2 cup mozzarella cheese, and bake for 5 more minutes or until cheese melts.