Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Line a large baking sheet with parchment paper for easier cleanup.
In a large bowl, combine the sliced chicken, peppers, and onions.
In a small bowl, whisk together the olive oil, chili powder, cumin, paprika, oregano, salt, pepper, and cayenne pepper if using.
Pour the oil and spice mixture over the chicken and vegetables. Toss everything together until the chicken and vegetables are evenly coated.
Spread the coated chicken and vegetables in a single layer onto the prepared baking sheet. Do not overcrowd the pan; use two pans if necessary to ensure proper roasting.
Bake for 20 to 25 minutes, stirring halfway through, until the chicken is cooked through and the vegetables are tender-crisp.
Remove from the oven. Serve the fajita mixture immediately with warm tortillas and your choice of toppings.
Notes
For best results, slice the chicken and vegetables into uniform strips so they cook evenly.
If you prefer a softer vegetable texture, you can add the vegetables to the pan 5 minutes before adding the chicken.
You can substitute chicken thighs for breasts if you prefer darker meat.