1. In large skillet, saute zucchini and onion in butter until onion is tender. Pour into baking dish (10 x 6 x 1-1/2 inch). Sprinkle with 1/4 cup Parmesan cheese; set aside.
2. In same skillet, brown ground beef; drain excess fat. Stir flour, salt, pepper, then ketchup into meat.
3. Spoon meat mixture evenly over zucchini.
4. Cover and bake in a 350 degrees F. oven for 20 minutes. Remove cover; top with 1/4 cup Parmesan cheese; bake, uncovered, an additional 5 minutes or until cheese is lightly browned.