(optional) Preheat oven to 180ºC . Slice eggplant into 1 cm thick rounds. Prepare three bowls, the first with the flour mixed with half tsp salt, the second with the egg whites mixed with the skim milk, and the third with bread crumbs mixed with remaining half tsp salt, oregano, and pepper. Dredge eggplant slices in flour, then egg and milk mixture, and then bread crumb mixture. Make a layer of coated slices in a casserole dish and sprinkle with 1 cup tomato sauce and quarter cup parmesan cheese. Make another layer of slices on the first layer, staggering slices so they don't completely overlap lower slices. Sprinkle second layer with remaining 1 cup tomato sauce and quarter cup parmesan cheese. Cover casserole and bake for 45 minutes. Optional: remove casserole and sprinkle with mozzarella cheese and return to oven for 5 minutes until cheese is melted.