In a glass dish or a sealable plastic bag, combine all the ingredients except for the meat. Generously season the steaks with pepper and place in the marinade, turning to coat. Cover the dish or seal the bag and refrigerate overnight.
Preheat the grill, setting the burners to high. Oil the grate.
Grill the steaks for about 5 minutes on each side for rare meat, or until the desired doneness. Note that to be tender, flap steak must be served rare.
Set aside on a plate and let rest for 5 minutes.
Serve with baked sweet potatoes and grilled onions.