Preheat broiler. In a large ovenproof skillet, cook beef, onion and garlic over medium heat until beef is no longer pink, breaking up beef into crumbles; drain. Stir in water and taco seasoning; heat through. Stir in 1 cup cheese. Remove from heat., In a bowl, mix mashed potatoes and remaining cheese; spread over beef mixture. Broil 4-6 in. from heat 5-6 minutes or until top is golden brown.