In large skillet, saute zucchini and onion in butter until onion is tender. Pour into baking dish (10x6x1 1/2"); sprinkle with 1/4 cup Parmesan cheese; set aside. In same skillet, brown ground beef; drain excess fat. Stir flour, salt, pepper, then ketchup into meat. Spoon meat mixture evenly over zucchini. Cover; bake in 350 oven 20 minutes. Remove cover; top with 1/4 cup Parmesan cheese; bake, uncovered, and additional 5 minutes or until cheese is lightly browned. Cut into squares to serve. Makes 6 servings.