Make the queso: Melt the butter in a saucepan over medium heat. Sauté the jalapeno pepper (if using) for about 3 minutes. Whisk in the flour and stir for about 1 minute until bubbly and golden. Gradually whisk in the milk. Add in the shredded cheese and paprika and whisk continuously until smooth. The sauce should start to thicken. Season with salt and pepper to taste. Once it's creamy and thick, remove it from heat and cover it with a lid.
Prep quesadilla ingredients per the instructions above.
Heat the oil in a saute pan on medium heat. Sauté the onion and peppers until they start to soften. Then add in the chicken, cajun seasoning, and season with salt. Continue to sauté about 5 minutes, until the chicken is browned and reaches an internal temperature of 160 degrees using a thermometer. Then remove the pan from the heat.
Preheat a panini-style grill or nonstick pan on medium-low heat.
Spread the cheese onto the tortilla and top with the chicken mixture, then fold the tortilla in half.
Place the queso over low medium heat. Stir occasionally, to gradually bring it up to being hot.
Cook the quesadilla on the panini grill or pan until golden brown on both sides and the cheese is melted.
Serve: Cut the quesadilla into wedges. Enjoy with sour cream (if using), queso with chips, and fruit punch (over ice if desired).