Preheat oven to 375 F. Cook ground beef in skillet until it loses redness.
Place potatoes and onions in buttered 9x13" pan. Add meat to mixture. Mix cheese soup and milk until smooth and add to meat and potatoes; mix together. Combine tomato sauce, salt, pepper, oregano and sugar. Pour sauce over the top, but do not mix. Dot with butter. Cover pan with foil. Bake at 375 F for 1 hour. Remove cover and arrange sliced cheese on top. Sprinkle with Parmesan.
Return to oven, uncovered, for about 15 minutes or until cheese bubbles.