Put the steak in a nonreactive dish and pour over the tequila, lime juice and olive oil. Turn to coat. Add the chili powder, and use your hands to work it into the meat. Let stand about 1 hour.
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.
Halve the avocados and scoop the flesh into a bowl. Add the remaining ingredients and use a fork to mash to a chunky mix.
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.
Recipe courtesy of Stephanie March