1. Brown together ground beef and onion until beef is brown and onion is tender; drain well.
2. Add the mushroom soup, cheddar cheese soup, Ro-tel tomatoes and soup can of milk to beef/onion mixture; stir until well mixed.
3. Line a 13x9 baking dish with the Doritos.
4. Cover with the meat/soup mixture and sprinkle completely with the cheddar cheese.
5. Bake in a preheated 350 degree oven for about 30 minutes or until heated through and cheese is melted.