Ingredients
1 1/4 to 1 1/2 pounds flank steak, patted dry and pierced at approximately 1 1/2-inch intervals with the tines of a fork
Kosher salt and freshly ground pepper
1/4 cup balsamic vinegar
1 tablespoon Worcestershire sauce
2 teaspoons ground mustard
2 tablespoons light-brown sugar
1 tablespoon extra-virgin olive oil, plus more for grill
3 rosemary sprigs
1 large sweet onion, such as Vidalia, cut into scant 1/2-inch-thick rounds
2 red or orange bell peppers, stems, ribs, and seeds removed, cut into scant 1/2-inch-thick rounds
Crusty bread, for serving (optional)
Preparation
Generously season steak with salt and pepper. In a large resealable bag, combine vinegar, Worcestershire, mustard, brown sugar, and oil. Rub rosemary sprigs between your palms for a few seconds to release oils, then transfer to bag. Add steak, onion, and bell peppers; remove excess air from bag, seal, and shake until marinade evenly coats meat and vegetables. Refrigerate at least 12 hours and up to 2 days.
Remove steak and vegetables from refrigerator 1 hour before grilling. Preheat grill for direct-heat cooking. Brush grates with oil. Using tongs, remove steak from bag, allowing excess marinade to drip back in. Grill, turning a few times, until charred in places and a thermometer inserted into thickest part registers 125 degrees for medium-rare, 8 to 12 minutes, depending on thickness. Transfer to a plate; tent with foil and let stand at least 10 minutes before slicing. Meanwhile, remove vegetables from marinade and grill, turning a few times, until charred in places and tender, 10 to 12 minutes. Toast bread on grill just until charred and crisp in places, about 1 minute a side. Thinly slice steak; serve with vegetables and bread.
