Heat a charcoal or gas grill until hot (you should barely be able to hold your hand 3 or 4 inches from the fire), or use a grill pan or medium-sized skillet on medium-high heat. Meanwhile, bring the steak to room temperature, pat it dry with paper towels, and sprinkle both sides with salt and pepper.
Grill steak until nicely browned, 3 to 4 minutes per side. Move the meat to a cooler part of the grill and cook another 5 to 7 minutes, turning once.
While steak is cooking, preheat oven to 325 degree F. Chop the tomato, shred the lettuce, and slice the avocado; wrap the tortillas in foil and put them in the oven to warm.
When steak is done, cover and let rest for 5 minutes before cutting across the grain into thin slices. Serve the steak in the tortillas, topped with tomato, lettuce, avocado, and a dollop of plain yogurt or light sour cream, if you like.