Preheat oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Slice the chicken breast into thin strips and place them on the sheet pan.
Slice the bell pepper and red onion into thin strips and add them to the pan with the chicken.
Drizzle the olive oil over the chicken and vegetables, then sprinkle with chili powder, cumin, garlic powder, sea salt, and black pepper.
Toss everything together until evenly coated and spread into a single layer.
Roast for 15-18 minutes until the chicken is cooked through and the vegetables are tender-crisp.
During the last 2 minutes of roasting, wrap the corn tortilla in foil and place it in the oven to warm.
Slice the avocado and squeeze the lime juice over the roasted chicken and vegetables.
Assemble the fajita by filling the warm tortilla with the chicken mixture and topping with the creamy avocado slices.