1. Combine first 7 ingredients in a large zip-top plastic bag; seal. Marinate in refrigerator 30 minutes.
2. Preheat broiler.
3. Remove steak from bag, reserving marinade. Place steak on a broiler pan coated with cooking spray; broil 5 minutes on each side or until desired degree of doneness. Remove steak from oven; loosely cover with foil.
4. While steak broils, place reserved marinade in a small nonstick skillet. Bring to a boil; cook until reduced to 1/3 cup (about 6 minutes), stirring occasionally.
5. Cut steak diagonally across the grain into 1/4-inch-thick slices. Drizzle mustard-molasses sauce over steak. Top with onions, if desired.