Source: Bobby Flay - Boy Gets Grill
adapted by There's Always Thyme to Cook
1/2 cup balsamic vinegar
3/4 cup olive oil
4 cloves garlic, coarsely chopped
2 tablespoons coarsely chopped rosemary leaves
1 (1 1/2- to 1 3/4-pound) beef flank steak
Salt and freshly ground pepper
Whisk together the vinegar, the oil, the garlic and rosemary in a large dish (or use a large Ziploc freezer bag). Add the steak and turn to coat. Cover and let marinate in the refrigerator for at least 4 hours and up to 8 hours, turning every 2 hours.
Heat your grill to high.
Remove the steak from the marinade and season with salt and pepper. Grill until lightly charred and crusty, 4 to 5 minutes. (Do not close lid on grill unless it is cold or windy) Turn steak over, reduce the heat to medium or move to a cooler part of the grill, and grill until medium-rare, 3 to 4 minutes more. Be careful not to overcook.
Remove the steak to a cutting board and let rest for 5 minutes. Cut into 1/2-inch thick slices against the grain of the meat.