Slice the chicken breast across the grain into strips about 1/2 inch thick, so they cook through in the time it takes to sear the outside.
Toss the chicken with the lime juice, 1 tablespoon of the oil, the chili powder, cumin, smoked paprika, garlic powder and salt. Leave it for 15 to 30 minutes while you slice the vegetables.
Heat a large cast iron skillet over high heat until it is smoking lightly. Add 1 tablespoon of oil and half the chicken in a single layer with space between the pieces.
Sear without moving for 2 minutes, turn, and cook another 2 to 3 minutes until browned and cooked through, 165F in the thickest piece. Move to a plate and repeat with the rest.
Add the last tablespoon of oil to the empty skillet, then the peppers and onion. Cook over high heat, tossing only occasionally, for 5 to 7 minutes until they are blistered and charred at the edges but still have some bite.
Return the chicken and any juices to the pan and toss for 30 seconds to rewarm.
Warm the tortillas in a dry pan for about 20 seconds a side. Serve the fajitas in the skillet with the tortillas and toppings on the side.