Heat oven to 400 degrees. Grease a 13×9 pan. Brown beef with onion, drain fat. Stir in sauce. Spoon sauce and meat mixture into pan and top with cheese slices. Beat eggs, milk, and oil until foamy. Beat in flour and salt until smooth. Pour batter over hot meat filling. Sprinkle with Parmesan cheese. Bake for 30 minutes or until golden brown.