(optional) Preheat oven to 350 degrees F. Slice eggplant into 1/4" thick rounds. Prepare three bowls, the first with the flour mixed with 1/2 tsp salt, the second with the egg whites mixed with the skim milk, and the third with bread crumbs mixed with remaining 1/2 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 1/4 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 1/4 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.