Brown 1 lb. ground beef with 2 tablespoons chopped red onion. Spread 1 bag tater tots in rectangle casserole dish. Spread browned beef over tater tots. In a bowl, mix 2 cans cream of mushroom soup and 1 can of milk. Pour over beef and tater tots. Grate 1 lb. Cheddar cheese and sprinkle on top. Bake at 350 degrees for 30 minutes.