Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Slice the chicken breast into thin, even strips and place them on the prepared baking sheet.
Add the sliced bell peppers and red onion to the sheet pan with the chicken.
Drizzle the olive oil over the chicken and vegetables, then sprinkle evenly with chili powder, cumin, garlic powder, sea salt, and black pepper.
Toss all ingredients together on the pan until thoroughly coated in oil and spices, then spread them out into a single layer.
Roast in the center of the oven for 18 to 20 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
While the fajitas roast, warm the corn tortilla in a dry skillet over medium heat for about 30 seconds per side until pliable.
Assemble the fajita by piling the roasted chicken and vegetables onto the warm tortilla.
Top with fresh avocado slices and a bright squeeze of lime juice before serving.