In a large mixing bowl, combine the sliced chicken, bell peppers, and onion.
In a small bowl, whisk together the olive oil, chili powder, cumin, smoked paprika, salt, and pepper. Pour this spice mixture over the chicken and vegetables.
Toss everything together until the chicken and veggies are well coated with the spices.
Spread the chicken and vegetable mixture in a single layer on a sheet pan lined with parchment paper.
Bake in the preheated oven for 20-25 minutes, or until the chicken is cooked through and the vegetables are tender. Stir halfway through for even cooking.
While the chicken and veggies are baking, warm the tortillas in a dry skillet over medium heat for about 30 seconds on each side or wrap them in aluminum foil and heat them in the oven for the last 5 minutes of cooking time.
Once done, remove the sheet pan from the oven.
Serve the sizzling chicken and veggies straight from the sheet pan, allowing guests to assemble their own fajitas in warm tortillas. Top with fresh cilantro, avocado slices, a squeeze of lime, sour cream, and shredded cheese as desired.