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 at least 30 minutes, and up to 4 hours.

2. Preheat broiler. Remove steak from marinade and transfer to a rimmed baking sheet. Season generously with salt and pepper on both sides. Place under broiler, and cook about 3 minutes. Turn over, and cook 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.