Combine marinade ingredients and 3 Tbsp. water. Marinate flat iron steaks in marinade in refrigerator 1 to 2 hours.
Remove steaks; discard marinade. Season with salt and pepper. Place steaks on grill over, ash-covered coals or gas grill preheated to high. Grill, uncovered, 6 minutes on the first side and 5 minutes after turning for medium rare to medium doneness.