Combine all the spices — chili powder, cumin, smoked paprika, garlic powder, onion powder, oregano, salt, black pepper, and cayenne (if using) — in a small bowl and stir until evenly blended. Set aside.
Add the sliced chicken to a medium bowl, drizzle with 1 tablespoon olive oil, add all the fajita seasoning, and squeeze the juice of half a lime over the top. Toss to coat and let marinate for at least 5 to 10 minutes (or refrigerate up to 4 hours).
Slice the bell peppers into ¼-inch strips and the onion into ¼-inch half-moon strips. Keep in a separate bowl from the chicken.
Heat a large cast iron skillet over medium-high to high heat, add 1 tablespoon olive oil, then cook the chicken strips in a single layer for 3 to 4 minutes per side until golden and cooked through to 165°F (74°C). Remove and set aside.
In the same pan, add the remaining 1 tablespoon olive oil and cook the onions and peppers over medium-high heat for 4 to 5 minutes, stirring occasionally, until tender-crisp with light char. Squeeze the remaining lime juice over the top.
Return the chicken to the pan with the peppers and onions and toss together over medium heat for about 1 minute to warm through and meld the flavors. Taste and adjust salt or lime juice as needed.
Warm the flour tortillas in a dry skillet for 30 seconds per side, over a gas flame with tongs, or wrapped in a damp paper towel in the microwave for 30 to 45 seconds. Keep warm in a tortilla warmer or wrapped in a kitchen towel.
Spoon the chicken and vegetable filling into warm tortillas, add your toppings of choice, and enjoy immediately.