Heat the oven: Preheat the oven to 400°F and line a large rimmed sheet pan with foil or parchment for easy cleanup.
Mix the seasoning: In a small bowl, stir together the chili powder, cumin, smoked paprika, garlic powder, salt, pepper, and cayenne if using.
Toss everything together: Pile the sliced chicken, bell peppers, and onion on the sheet pan. Drizzle with the olive oil, sprinkle the seasoning over the top, and toss with your hands until everything is evenly coated.
Spread it out: Spread the chicken and vegetables into a single layer with a little space between them so they roast instead of steam. Use two pans if it looks crowded.
Roast: Roast for 20-22 minutes, tossing once halfway through, until the chicken reaches 165°F on a thermometer and the peppers and onions are tender with charred edges.
Finish and serve: Squeeze the lime juice over the pan and scatter with cilantro. Serve right away with warm tortillas and your favorite toppings.
Notes
Spread the chicken and vegetables in a single layer so the hot oven chars them instead of steaming them. Cook the chicken to an internal temperature of 165°F.