Preheat your oven to 400°F (200°C).
Slice the chicken breast, bell peppers, and red onion into thin, even strips.
Place the chicken and vegetables onto a large parchment-lined sheet pan.
Drizzle with olive oil and sprinkle the chili powder, cumin, garlic powder, sea salt, and black pepper over the top.
Toss everything together with your hands or tongs until the chicken and vegetables are thoroughly coated in oil and spices.
Spread the mixture into a single layer to ensure even roasting.
Bake for 18 to 20 minutes until the chicken is cooked through and the vegetables are tender with slightly charred edges.
While the fajitas roast, warm the corn tortillas in a dry skillet over medium heat for 30 seconds per side.
Divide the chicken and vegetable mixture between the tortillas.
Garnish each fajita with sliced avocado, fresh cilantro, and a squeeze of lime juice before serving.