Balsamic-Marinated Flank Steak

3/4 lb flank steak
1/3 cup finely chopped onion
1 garlic clove, minced
3 tbs balsamic vinegar, divided
2 tsp olive oil
1/2 tsp pepper
1/4 tsp salt
Score both sides of flank steak in cross-cross pattern, cuttin 1/8-inch deep. Place in shallow glass baking dish. Combine onion, garlic, 2 tbs of the vinegar, oil and pepper in small bowl. Pour over steak, turning to evenly coat both sides. Cover and refrigerate at least 4 hours or overnight. Heat grill. Remove steak from marinade. Discard marinade. Sprinkle both sides of steak with salt. Grill, covered, over medium heat or coals 9 to 12 minutes or until internal temperature reaches 140 degrees F for medium-rare, turning once. Place on cutting board. Let stand 5 minutes. Cut diagonally into thin slices. Pour accumulated juices over slices. Drizzle with remaining 1 tbs vinegar. 2 servings = about 255 calories.