Brown beef and onion in frying pan. Sprinkle with salt and pepper. Stir. Transfer to bottom of 2- quart casserole. Break up spaghetti for easier serving. Cook according to package directions. Drain. Layer over meat. Break up large tomato chunks. Pour over top. Spoon soup over tomatoes. Cover with cheese. Bake uncovered in 350 F oven for 30 minutes until hot and and cheese is melted. Cover half-way through cooking if cheese starts getting dry.