DO-AHEAD TIP: Marinate steak for 20 to 30 minutes (see recipe).
Trim steak of excess fat. Rub with garlic and black pepper then place it in a glass baking dish. Pour the vinegar over the steak.
Cover and marinate at room temperature for 20 to 30 minutes, turning every 5 minutes. Meanwhile, preheat grill to MEDIUM-HIGH. Remove steak from marinade (reserving marinade) and pat dry with paper towels.
Grill to desired level of doneness then transfer to a cutting board and let stand for 15 minutes. In the meantime, pour reserved marinade into a medium saucepan over medium heat.
Cook until mixture becomes syrupy. Thinly slice steak across the grain (diagonally) and serve with balsamic sauce. Enjoy!!!