Brown beef with onion in electric skillet. Drain off fat. Stir in water, rice, crushed bouillon cubes and dry mustard. Bring to a boil (at 375 degrees). Reduce heat to 250 degrees, cover and simmer until liquid is absorbed about 25 minutes. Stir in green pepper and tomato. Sprinkle cheese over top. Cover and turn off the heat. Let stand 2-3 minutes to melt cheese. Makes 4-6 servings