Preheat the oven to 400 F. Lightly grease a cookie sheet with non-stick cooking spray.
In a large skillet, saute the onion and peppers in the olive oil for 4 to 6 minutes. Once softened, mix in the chopped chicken and taco/fajita seasoning. Let simmer for a few minutes to allow flavors to combine.
Lay the tortilla on the baking sheet and sprinkle the shredded cheese blend onto half of the tortilla. (You will be folding the tortilla, so sprinkle a thin layer over half.) Top with the chicken and veggies mixture. Then sprinkle more shredded cheese on top of the beef. Fold the tortilla over to cover the cheese.
Repeat with the remaining 3 tortillas, chicken and shredded cheese. 4 half-folded tortillas should fit on a standard size cookie sheet.
Bake in the preheated oven for 8 minutes, then carefully flip and cook another 5 minutes. You want both sides to be slightly golden and the cheese melted through.
Run a knife of pizza wheel through the quesadilla and cut into wedges.
Serve Chicken Fajita Quesadillas with side of fruit and/or salad.