Place the flank steak in a large, resealable plastic bag. In a 2-cup measuring cup, combine the remaining ingredients. Pour the marinade mixture into the bag with the steak, seal, and place in the refrigerator for at least 4 hours or up to 12 hours.
Preheat grill. Remove the steak from the marinade. Pour the marinade into a small saucepan and bring to a boil. Lower the heat to medium-low and simmer for 10 minutes. Remove from heat but keep warm.
Grill the steak for 5-6 minutes per side or to desired doneness. A meat thermometer should read 125°F for rare, 135°F for medium rare, and 145°F for medium.
Transfer the steak to a plate and let stand for 5 minutes before carving. Cut the steak across the grain into thin diagonal slices. Serve with the marinade sauce.
