Heat the oven to 375°F and grease a 9-by-13-inch baking dish. Boil the macaroni 2 minutes short of package directions and drain.
Brown the beef and onion in a large skillet, breaking the meat into small crumbles. Drain excess fat, add the garlic and cook for 30 seconds.
Sprinkle in the flour and cook for 1 minute. Slowly stir in the milk and broth, then add mustard, Worcestershire, salt and pepper and simmer until lightly thickened.
Remove from the heat and stir in 2 cups cheddar and the macaroni. Spread in the dish and top with the remaining cheese.
Bake for 25 to 30 minutes, until bubbling at the edge. Rest for 10 minutes before serving.
