Prep the produce and chicken. Thinly slice the peppers and onion. Slice the chicken into thin strips, about 1/2 inch wide, so it cooks quickly and evenly.
Mix the spices. In a small bowl, combine chili powder, cumin, paprika, garlic powder, onion powder, oregano, crushed red pepper (if using), salt, and pepper.
Season the chicken. Pat the chicken dry with paper towels. Toss it with half of the spice mix and 1 tablespoon of olive oil until evenly coated.
Sear the chicken. Heat a large skillet (cast iron is great) over medium-high heat.
Add the seasoned chicken in a single layer. Cook for 5–7 minutes, stirring once or twice, until the edges are slightly charred and the chicken is cooked through. Transfer to a plate.
Cook the veggies. In the same skillet, add the remaining 1 tablespoon of olive oil.
Add onions and peppers with a pinch of salt. Cook for 5–7 minutes, tossing occasionally, until tender-crisp with a bit of color.
Combine and finish. Return the chicken to the skillet. Sprinkle the remaining spice mix over everything and toss to coat.
Squeeze in the lime juice and cook 1–2 minutes more to meld the flavors. Adjust salt and pepper to taste.
Warm the tortillas. Briefly heat tortillas in a dry skillet over medium heat for 20–30 seconds per side, or wrap in a damp towel and microwave for 20–30 seconds until soft.
Serve. Fill warm tortillas with the chicken and peppers. Add your favorite toppings and a sprinkle of cilantro, if using.
Serve immediately.