2) Preheat the grill to high. Remove the steak from the marinade and season with salt and pepper. Grill, covered, turning once, 3 to 4 minutes per side for medium rare.
3) Halve the avocados and scoop the flesh into a bowl. Add the remaining ingredients and use a fork to mash to a chunky mix.
4) Remove the steak from the grill and rest 5 to 10 minutes. Slice across the grain. Heat the tortillas to soften. Fill the tortillas with steak, lettuce, tomato, onion and guacamole.