For time-savings, use precooked, diced rotisserie chicken in place of raw chicken.
Cut each chicken breast lengthwise in 3 strips; cut crosswise into thin pieces. Combine with the fajita seasoning and mix well.
Heat the oil in a large skillet over medium heat. Add the chicken and cook 7 to 8 minutes or until no longer pink in the center, stirring frequently. Stir in the bell peppers and onion. Cook the vegetables for 5 to 7 minutes or until the vegetables are tender.
For each quesadilla, sprinkle 1/4 cup Cheddar cheese on half of a tortilla. Spoon about 1/2 cup of the chicken mixture over the cheese. Top with 1/4 cup Monterey Jack cheese and fold the tortilla over, forming a half moon. Press lightly.
Spray a skillet or griddle with cooking spray; warm to medium heat. Lightly brown each quesadilla for 2 to 3 minutes per side until the cheese is melted.