Using a tenderizing tool, pierce flank steak on both sides; set aside.
Place all marinade ingredients in a small bowl and whisk until slightly thickened.
Pour marinade into a large zipper freezer bag and add the steak. Turn bag several times to coat the meat. Place in the refrigerator and marinate at least 2 hours, preferably overnight.
Heat outside BBQ grill (or indoor broiler) to high heat. Cook steak 8 minutes each side for rare, 10 minutes each side for medium. FYI flank steak is too tough to do well done IMO.
Remove steak from grill and tent with foil. Let rest 10 minutes before slicing. Remember to slice across the grain in about 1/2-inch slices cutting on an angle for best presentation.