Rub the steak with chili powder (if you like) and if it came from the fridge, let stand at room temperature for 10-15 minutes.
Heat a drizzle of oil in a heavy skillet (cast iron is ideal) set over medium-high heat, and once the pan is hot, cook the steak for 5 minutes, until it develops a deep bottom crust. Flip and cook for another 5-6 minutes for medium-rare, or until done to your liking.
Transfer to a cutting board and let rest for 10 minutes. Slice the steak thinly across the grain.
Serve in tortillas with guacamole, salsa, crumbled feta, cilantro and lime wedges for squeezing overtop.