Rinse and dry the flank steak, use a 50/50 vinegar/water mixture. Score the meat lightly in a 2-in (5 cm) diamond pattern to allow the flavour of the marinade to penetrate. Combine marinade ingredients in a heavy resealable bag. Place meat in the marinade, seal and marinate in the refrigerator 6 hours or overnight, turn 2-3 times to expose the meat thoroughly.
Preheat your Broil King® on HIGH, season or brush the grids with oil to prevent sticking. Place the flank steak on grids, reduce heat to MEDIUM. Grill approximately 5 minutes per side for medium. Allow the steak to rest for 3-5 minutes before carving.
Meanwhile, pour the marinade into a small saucepan, bring to a boil on the side burner, boil for 2-3 minutes to reduce.
Carve the steak into thin slices diagonally, against the grain. Drizzle the marinade reduction over the meat.