Grilled Balsamic Sirloin Steak

Whisk the balsamic vinegar, olive oil, garlic, rosemary, mustard, salt, and pepper in a shallow dish. 2. Add the steak, turn to coat, and marinate in the refrigerator for at least 30 minutes or up to 8 hours. 3. Heat the grill to medium-high and oil the grates. 4. Grill the steak 5-6 minutes per side for medium-rare, until an instant-read thermometer reads 130F. 5. Move the steak to a cutting board, tent loosely with foil, and rest 8-10 minutes. 6. Slice thin against the grain and spoon over any resting juices.