In a large, oven safe skillet over medium heat, saute the beef for 2-3 minutes or until some of the fat is rendered. Add the onion and continue to cook until beef is browned and onion is softened. Add the garlic and saute until fragrant, about 1 minute. Drain off any excess fat, then season the beef to taste with salt and pepper.

Stir in the beef broth, canned tomatoes, and ketchup. Turn the heat up to high and bring to a boil. Add the macaroni noodles and cook for 7-8 minutes, stirring occasionally, until liquid is absorbed and noodles are al dente.

Lower the heat, then stir in the mustard, milk, and 2 cups of shredded cheese until melted and creamy. Sprinkle the remaining cheese on top then broil for 3-4 minutes or until browned and melty. (optional)