1/4 cup balsamic vinegar
2 tsp. Dijon mustard
1 tsp. salt
1 tsp. rosemary, crumbled
1 tsp. olive oil
1 1/4 pounds beef flank steak
3 medium onions, cut into 1/2-inch slices
Combine all ingredients in a 9x13 pan and turn several times to coat all. Let stand at least 15 minutes.
Prepare outdoor grill, or heat a raised ridge grill pan. Grill steak and onions until done, about 8 to 12 minutes, turning once and brushing with marinade. To serve, slice steak against grain and serve steak slices with onions.