In a small bowl combine all marinade ingredients, mixing well. Score the flank steak on both sides, by slicing about ¼ inch to form a cross-cross pattern. Place the steak in a glass baking dish if making right away, pour marinade over the meat. Cover. Refrigerate at least 4 hours or overnight, turning once during the marinade time.
Heat grill to medium hot, then reduce to low heat, place meat on grill and grill for 6-8 minutes per side (depending on thickness of flank steak and desired doneness.) Basting with leftover marinade if desired. For grill marks, rotate beef ¼ turn after 3 minutes, repeating on other side.
Remove meat from grill and cover with foil allowing to rest for about 6-7 minutes before slicing. Slice thinly against the grain. Serve immediately.
Recipe easily doubles. Label gallon size baggie prior to filling with meat and marinade. Place meat in freezer baggie, pour marinade over meat, remove as much air as possible and lay flat to freeze. When ready to barbecue, take out of freezer allowing to thaw in the refrigerator overnight, turning once during thawing. Cook according to above directions.