Preheat the oven to 425°F and line a large baking sheet with parchment paper.
Place the chicken, peppers, onions, garlic and olive oil on the baking sheet.
Combine the ingredients for the seasoning in a small bowl, then sprinkle evenly over the chicken and vegetable mixture on the baking sheet.
Toss everything together with tongs or your hands to evenly coat. Spread into an even layer.
Cook in the preheated oven for 15-20 minutes, tossing once halfway through, until the chicken is cooked through and the vegetables are tender. In the last 5 minutes of cooking, tightly wrap a stack of tortillas in foil and place it on the baking sheet with the fajita mixture to warm them.
Serve the fajitas with the warmed tortillas, along with lime juice, cilantro, and sliced avocados, if desired.
