Heat the grill to medium-high heat. Remove the meat from the marinade. Grill for 3 minutes per side.
Remove from the grill, and let rest for 10 minutes. Slice the beef against grain.
Serve with tortillas, onions, cabbage and limes.
When trimming a whole flat iron, cut the beef horizontally. There is a membrane that runs down the center of the cut. Removing this membrane ensures that every bite is taco tender.