Prepare marinade: In a large zipper-topped plastic bag, place the juice of 3 limes, the chopped cilantro, onion and salt and pepper. Add flank steak, seal bag and turn to coat; refrigerate overnight or for at least 8 hours.
Preheat outdoor or indoor grill or oven broiler to HIGH. Grill/broil steak for 4 to 6 minutes per side then transfer to a cutting board and let it rest for 5 minutes.
Thinly slice steak across the grain (diagonally). Cut remaining limes in half and set aside. Fill Romaine lettuce leaves with steak strips, shredded lettuce, tomatoes, cheese and salsa then squeeze lime juice on top; roll up and enjoy!