Place chicken in a small bowl; sprinkle with fajita seasoning, tossing to coat.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray.
Add chicken and onion; stir-fry 3 minutes.
Add bell pepper; stir-fry 5 minutes or until chicken is done.
Place 1 tortilla on each of 4 plates. Top each evenly with chicken mixture and salsa. Top with avocado and cilantro, if desired. Fold tortillas over filling, and top with sour cream, if desired.