Trim steak of excess fat. Rub steak with garlic and pepper. Place steak into a glass baking dish and pour vinegar over steak. Marinate steak for 20-30 minutes, turning the steak every 10 minutes. Remove steak from marinade and pat dry with paper towel.
Grill or broil steak to desired degree of doneness. Pour marinade into a saucepan and simmer over medium high heat until the liquid is syrupy. Add 2 tablespoons cold butter to syrup and swirl to melt butter. Slice steak across the grain in thin slices and serve with balsamic sauce