Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Line a large, rimmed baking sheet with parchment paper or aluminum foil for the easiest cleanup.
In a large bowl, combine the sliced chicken, peppers, and onion.
In a small bowl, whisk together the olive oil, chili powder, cumin, smoked paprika, garlic powder, oregano, salt, and pepper.
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 18 to 22 minutes, or until the chicken is cooked through (internal temperature reaches 165 degrees Fahrenheit) and the vegetables are tender and slightly charred.
Remove the pan from the oven. Serve the hot fajita filling immediately with warm tortillas and your choice of toppings.
Notes
For a stovetop alternative, cook the mixture in a large skillet over medium-high heat, stirring occasionally, until the chicken is done, about 10 to 12 minutes.
If you prefer a spicier flavor, add 1/4 teaspoon of cayenne pepper to the spice mix.
Warm your tortillas briefly in the oven during the last 5 minutes of cooking or wrap them in a damp paper towel and microwave for 30 seconds.