In a large skillet, cook the ground beef and onion until no longer pink. Drain grease, and return the beef to the pan.
Add the beef broth, ketchup, mustard, salt, and pepper, and uncooked elbow noodles. Boil, cover, and reduce heat to a simmer. Cook for 10-12 minutes then uncover and cook for 3-5 minutes.
Add the sour cream, milk, and 2 cups of cheese.
Stir well until the cheese melts and the ingredients are incorporated.
Top with remaining cheese, tomatoes, and lettuce. Serve hot!
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