Preheat your oven to 425°F and line a large sheet pan with parchment paper for easy cleanup. This temperature is hot enough to create that signature charred, restaurant-style finish on the vegetables and chicken without drying anything out.
In a small bowl, combine smoked paprika, cumin, chili powder, garlic powder, salt, and black pepper. Whisk these spices together thoroughly so the flavors distribute evenly throughout your sheet pan fajita mixture rather than clumping in certain spots.
Place sliced chicken breast on one half of the sheet pan and sliced peppers and onions on the other half. Drizzle everything with olive oil and the spice mixture, then toss each section separately so the chicken and vegetables get fully coated with those smoky flavors.
Roast for 15-18 minutes, stirring halfway through cooking, until the chicken reaches 165°F internally and vegetables develop light char marks. Sheet pan chicken fajitas are ready when the chicken is no longer pink and the peppers have softened with caramelized edges.
Remove from the oven and immediately squeeze fresh lime juice over everything. Let it rest for 2 minutes so the juices redistribute and flavors meld together beautifully.
Warm your tortillas in a dry skillet for 30 seconds per side, then fill each one with the chicken and vegetable mixture. Top with a dollop of sour cream and fresh cilantro, then serve immediately while everything is hot and aromatic.