In small bowl, combine chicken, shredded cheese, bell pepper and onion, if desired.
Evenly divide chicken mixture over each of 4 tortillas. Top with remaining flour tortillas. Lightly spray both sides of each quesadilla with nonstick cooking spray. Place quesadillas onto baking sheets.
Bake at 400° for 12 minutes or until golden brown, turning once half-way through heating.
Cut quesadillas into wedges and serve with sour cream, guacamole, salsa and lettuce.