Combine the marinade ingredients in a liquid measuring cup. Place the flank steak in a resealable plastic bag and add the marinade, tossing to coat. Place the bag in the refrigerator and marinate for a minimum of 6 hours to overnight, turning the bag periodically.
Remove the bag from the refrigerator and allow to steak to come to room temperature before grilling. When you’re ready to cook, preheat the grill to the highest temperature. Discard the marinade and pat the steak dry. Season both sides generously with salt and pepper.
Grill the steak on high heat for approximately 3-4 minutes per side until it’s cooked to medium-rare. Allow the meat to rest for at least 10 minutes. Slice against the grain in to thin strips. Serve topped with salsa.