Add the flank steak to the zip-top bag with the marinade and place it in the refrigerator to marinate for 4 hours.
Preheat the grill to medium heat. Remove the steak from the marinade, letting most of the marinade drip off. Season with salt and pepper on both sides and place it onto the preheated grill.
Cook for approximately 6 to 8 minutes per side, or until desired doneness (130°F-135°F for medium-rare, around 140°-145°F for medium).
Remove from grill to a cutting board, tent it with foil and let rest for 10 minutes.
Slice the steak against the grain and serve, topped with the fresh cilantro.