Brown ground beef and onion. Drain. Add 1 can tomato soup and 1/2 can onion rings. Put into casserole dish. Top with a ring of mashed potatoes and cheddar cheese. Bake at 350F for thirty minutes. Sprinkle the other 1/2 can of onion rings on the top and bake an additional 5 minutes or until they begin to brown a bit.