In a large bowl, combine the sliced chicken, red and green bell peppers, and onions.
Drizzle olive oil over the mixture, then sprinkle with fajita seasoning, lime juice, salt, and pepper. Toss well to ensure everything is evenly coated.
Spread the chicken and vegetable mixture evenly on a large sheet pan.
Bake in the preheated oven for about 20 minutes, or until the chicken is cooked through and the vegetables are tender. Stir halfway through the cooking time for even roasting.
Once done, remove from the oven and let it rest for a couple of minutes.
Serve the chicken and vegetable mixture warm on tortillas, garnished with fresh cilantro, and offer sour cream and avocado on the side if desired.
Notes
Feel free to customize with your favorite toppings.