Lay the flank steak in a large baking dish and pour the Mojo over it. Wrap tightly in plastic wrap and refrigerate for 1 hour or up to 8 hours. Don't marinate the steak for more than 8 hours.
Preheat an outdoor grill or a ridged grill pan over medium-high flame. Brush the grates with a little oil to prevent the meat from sticking. Pull the steak out of the Mojo marinade and season the steak on both sides with salt and pepper. Grill the steak for 7 to 10 minutes per side, turning once, until medium-rare. Remove the steak to a cutting board and let it rest for 5 minutes. Thinly slice the steak across the grain on a diagonal.
Warm the tortillas for 30 seconds on each side in a dry skillet or on the grill, until toasty and pliable.
To make the tacos, stack up 2 of the warm tortillas, lay about 4 ounces of beef down the center, and sprinkle with some lettuce, onion, and cheese. Top each taco with a spoonful of the Salsa Fresca and garnish with lime wedges. Repeat with the remaining tortillas.