These flank steak fajitas make for a tasty, healthy dinner in a pinch. Preheated sheet pans sizzle the meat and vegetables just like in a skillet, but with a whole lot more hands-off time.
Position racks in upper and lower thirds of oven and place a large rimmed baking sheet on each; preheat to 500 degrees F.
Combine oil, chili powder, garlic powder, cumin and salt in a large bowl. Rub steak with half of the spice mixture. Add onions, bell peppers and poblanos to the bowl and toss to coat. Carefully place the steak on the pan on the top rack. Carefully spread the vegetables on the pan on the lower rack. Roast until the steak and vegetables are starting to brown, about 8 minutes.
Flip the steak and stir the vegetables. Turn the broiler to high and continue cooking the steak to desired doneness (an instant-read thermometer inserted in the thickest part will register 120 degrees F for medium rare) and the vegetables until charred, about 6 minutes more. Transfer the steak to a cutting board and let rest for 5 minutes.
Slice the steak across the grain and serve in tortillas with the vegetables and guacamole. Serve with lime wedges and cilantro, if desired.