1. In 4-quart Dutch oven cook beef, onion, celery, and garlic over medium heat until meat is browned and vegetables are tender; drain off fat. Sprinkle flour on beef mixture; cook and stir 2 minutes. Stir in broth and potatoes. Bring to boiling, stirring occasionally. Reduce heat. Simmer, covered, 10 minutes or until potatoes are tender.
2. Stir in tomatoes, cheese, tomato paste, ketchup, and mustard. Cook and stir until cheese is melted and smooth and soup just comes to gentle boiling. Stir in milk; heat through. Serve with toasted buns and cheeseburger toppings. Makes 6 main-dish servings.