Coat a large nonstick skillet with cooking spray and preheat it over medium-high heat or preheat the grill.In a small bowl, stir together the chili powder, salt, and pepper. Rub the spice mixture into both sides of the steaks.
Grill or broil the steaks for 4 minutes per side, turning once, for medium rare or to your desired degree of doneness. Remove from the grill and let the meat rest for 5 minutes then carve into thin slices.
Warm the tortillas by placing them on the grill for about 30 seconds, turning once (this step is optional). Place the carved steak, warm tortillas,sour cream, guacamole, cilantro,onion, and lime wedges, in serving dishes and let diners make their own tacos at the table.
To Make the Guacamole
Using a fork, Mash avocado in a medium bowl until chunky. Next add the chopped onions, cilantro, and lime, then combine until fully mixed. Place in a small bowl and serve with tacos.