Saute beef, onion, celery and garlic over med. heat until meat is browned and veggies tender; drain off fat.
Sprinkle flour into beef mixture, stir and cook 2 minutes. Add broth and potatoes and bring to a boil, stirring occassionally. Reduce heat and simmer, covered, for 10 minutes or until potatoes are done.
Add tomatoes, cheese, tomato paste, ketchup, and mustard. Cook until cheese is melted, soup is smooth and just comes to a gentle boil. Stir in milk and heat through.
Serve with toasted buns and toppings of your choice!