Make the marinade by blending all ingredients in food processor, blender or hand immersion blender.
Season the steak with plenty of Kosher salt and freshly ground pepper. Coat the steak on all sides with the marinade and let sit for at least one hour, but preferably 12 hours before grilling.
Prepare the grill for high-heat cooking.
Wipe excess marinade from the meat (if preferred), then place the flap steak over direct heat and grill for about 4 to 6 minutes per side, or until the internal temperature reaches 130° F for medium-rare or 140° F for medium.
Remove from the grill and rest on a cutting board for 10 to 15 minutes before slicing against the grain to serve.
You can substitute flank steak or skirt steak and use the same marinade.