To make the fajita seasoning, in a small bowl, whisk together chili powder, cumin, paprika, garlic powder, onion powder, cayenne pepper, and salt (I like 1 teaspoon).
To marinate the chicken, in a large zipper-top plastic bag, add chicken, cilantro, lime juice, and fajita seasoning. Mash until chicken is evenly coated. Marinate at least 30 minutes at room temperature or up to 1 hour in the refrigerator.
Preheat oven to 425 degrees. Line a rimmed baking sheet with foil or parchment for easy cleanup.
In a large bowl, add bell peppers and onions. Add olive oil, 1 teaspoon salt, and 1/2 teaspoon pepper and toss to coat. Transfer to prepared baking sheet and arrange in an even layer.
Place chicken on top of the peppers and onions. Roast until chicken is golden and crisp and reaches 165 degrees on an internal thermometer, about 20 to 30 minutes.
Transfer chicken to a cutting board and rest for 5 minutes before cutting into strips. Serve with peppers and onions, flour tortillas, sour cream, and hot sauce (or your favorite toppings).