In a saucepan, cook beef over medium heat until no longer pink; drain. Stir in soup and soup mix; heat through. Stir in cheese until melted. Place about 1/3 cupful on each bun.
In a saucepan over medium heat, cook beef and onion until beef is no longer pink; drain. Add soup, ketchup, mustard, salt and pepper; mix well. Simmer, uncovered, for 5-10 minutes. Serve on buns.