In a small bowl, add chili powder, cumin powder, paprika, cayenne pepper, garlic powder, salt, and black pepper. Mix together.
In a big bowl, season chicken with the spice mixture, about 2 teaspoons. Let marinate for 15 minutes.
Heat the cast iron pan with 2 tablespoon of olive oil on medium heat. Sear the chicken on both sides until golden, charred and cooked right through (about 6-8 minutes each side). Transfer chicken to a plate, slice into strips.
Heat the pan with olive oil, add bell pepper, onion. Stir-fry until the veggies are tender. Season with the spice blends and lime juice.
Warm tortillas in the oven over low heat (285°F) for 3 minutes.
Fill the tortillas with the chicken, avocado and veggies mixture above. Add more of your favourite toppings.