Preheat oven to 350 degrees. Meanwhile brown ground beef and onion in skillet; drain. Salt and pepper to taste. Place in the bottom of a casserole dish. In a bowl empty the can of mushroom soup and whisk in a half can of water. Pour over ground beef. Combine the cheeses and cover the soup using all of the cheese. Cover the top with tater tots. Bake at 350 for 45 minutes or until tater tots are brown and casserole is bubbly. Serve with saled or coleslaw and bread if desired.