Preheat the oven to 400°F. Line a large rimmed baking sheet with parchment paper.
Place the sliced chicken, bell peppers, and onion on the prepared baking sheet.
In a small bowl, stir together the olive oil, fajita seasoning, salt, and black pepper.
Pour the seasoning mixture over the chicken and vegetables. Toss until everything is evenly coated, then spread into a single layer.
Bake for 20–25 minutes, tossing halfway through, until the vegetables are tender and the chicken is cooked through to an internal temperature of 165°F.
Divide the chicken and vegetables among the warm corn tortillas and serve immediately.