1. Combine balsamic vinegar, olive oil, rosemary, and garlic in a glass or plastic dish, and stir to combine. Add skirt steak, and coat well with marinade. Cover, and refrigerate for at least 30 minutes, and up to 2 hours.
2. Preheat broiler or grill. Remove steak from marinade, and transfer to a rimmed baking sheet. Season meat generously with salt and pepper on both sides. Place under broiler or on grill, and cook about 3 minutes, turn over, and cook for 3 minutes more for medium rare. 3. Transfer to a cutting board to rest for 5 to 10 minutes. Slice thinly across the grain, and serve.