Preheat the oven to 425°F (220°C), or 200°C if using a fan oven.
In a small bowl, combine the seasoning ingredients and mix (chili powder, ground cumin, garlic powder, onion powder, smoked paprika, cayenne, oregano, black pepper). That’s your fajita seasoning ready. Set aside.
To a large rimmed sheet pan (I use a half sheet size 18″ x 13″, 46 x 33 cm), add oil, sliced chicken, sliced bell peppers, and sliced onion. Season with the fajita seasoning, salt, and toss with kitchen tongs or your hands until everything is well coated.
Roast in the oven for 10 minutes, flip, and then roast for 5 more minutes (the internal temperature of the chicken must be 165°F/74°C). Place under the broiler for 2-3 minutes to slightly char the chicken and vegetables. If desired, wrap the tortillas in foil, and add them to the oven in the last 3 minutes so they are warmed up.
Remove from the oven, allow to cool for a few minutes then serve in flour tortillas, with a squeeze of fresh lime juice, and your favorite toppings.