In a mixing bowl, combine the shredded leftover chicken, diced vegetables, cheese, and taco seasoning (if using). Mix well until all ingredients are evenly distributed.
Heat a large skillet over medium heat and lightly coat it with olive oil or cooking spray.
Place one tortilla in the skillet and spoon a generous amount of the chicken mixture onto one half of the tortilla. Fold the other half over to create a half-moon shape.
Cook the quesadilla for about 3-4 minutes on one side until golden brown and crispy. Carefully flip the quesadilla and cook for an additional 3-4 minutes on the other side.
Remove the quesadilla from the skillet and let it cool for a minute before slicing it into wedges. Repeat the process with the remaining tortillas and filling.