In an ovenproof skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Add garlic; cook 1 minute longer. Stir in water and taco seasoning; heat through. Stir in 1/2 cup cheese.
Combine potatoes and remaining cheese; spread over beef. Broil 4-6 in. from the heat for 5-6 minutes or until golden brown.