Combine sliced chicken with half the spices (paprika, cumin, oregano, garlic powder), lemon juice, olive oil, salt, and pepper in a large bowl. Toss to coat evenly.
Add sliced peppers and onion with the remaining spices to the bowl and toss again for even seasoning. Marinate 15–30 minutes if desired.
Preheat the air fryer to 375°F (190°C).
Arrange chicken and vegetables in the air fryer basket in a loose, even layer; work in batches if needed.
Air fry for 25 to 30 minutes, shaking halfway through, until chicken is cooked and edges are lightly charred.
While cooking, mix parsley, olive oil, lemon juice, garlic, red hot pepper, chili flakes, salt, and pepper to make the chimichurri sauce. Let sit 5–10 minutes.
Serve the hot chicken and vegetables topped generously with chimichurri sauce.