Preheat the oven to 425°F and lightly oil a large rimmed sheet pan. Pat the chicken dry and slice against the grain. Slice the peppers and onion into similar thickness for even cooking.
Add the chicken, peppers, and onion to the pan. Drizzle with olive oil and sprinkle with the fajita seasoning. Toss to coat, then spread into a roomy single layer.
Bake on the top rack for 20 to 25 minutes until the chicken is opaque and the vegetables are tender with caramelized edges. For more char, broil for 1 to 2 minutes at the end.
Wrap the tortillas in foil and warm them in the oven during the last few minutes. Squeeze fresh lime over the fajitas, sprinkle with cilantro, and serve hot with guacamole and pico de gallo.
Notes
If using thighs, slice into strips; they may need more time. For best browning, use a large pan and spread everything out.