Prep the chicken. In a bowl, toss the chicken with the BBQ sauce until it’s well coated.
Taste and add a pinch of salt or pepper if needed. Set aside while you heat the pan.
Warm the skillet. Heat a large nonstick skillet over medium to medium-low. Add a small amount of butter or oil to lightly coat the surface.
You want the tortilla to crisp without burning.
Build the first quesadilla. Place one tortilla in the skillet. Sprinkle 1/2 cup cheese over half the tortilla. Add a quarter of the BBQ chicken, then scatter a few onion slices, jalapeño, and cilantro if using.
Top with another small sprinkle of cheese to act like “glue.”
Fold and cook. Fold the tortilla over to cover the filling. Press gently with a spatula. Cook 2–3 minutes until the bottom is golden and the cheese starts melting.
Flip carefully and cook another 2–3 minutes until crisp and fully melted.
Repeat. Transfer the quesadilla to a cutting board and let it rest for 1 minute before slicing. Repeat with the remaining tortillas and filling, adding more butter or oil as needed.
Serve. Slice into wedges. Serve hot with sour cream, salsa, lime wedges, or a drizzle of ranch or chipotle mayo.
A quick side salad or grilled corn rounds out the meal.