Prep the chicken: In a bowl, toss the cooked chicken with BBQ sauce until coated but not dripping.
You want it saucy enough to flavor the quesadilla without making it soggy.
Heat the pan: Set a large nonstick or cast-iron skillet over medium heat. Add butter or oil and let it melt and coat the surface.
Build the base: Place one tortilla in the skillet. Sprinkle half the cheese evenly over the tortilla, going close to the edges.
Cheese at the edges helps seal the quesadilla.
Add the filling: Spread the BBQ chicken evenly over the cheese. Scatter red onion, jalapeño (if using), and half the cilantro. Add a small pinch of salt and pepper.
Top with the remaining cheese.
Close it up: Place the second tortilla on top and gently press down with a spatula so the cheese starts to melt and stick.
Cook until golden: Let it cook 3–4 minutes, checking the underside. When the bottom is golden and crisp and the cheese is melting, carefully flip using a wide spatula. If flipping is tricky, slide the quesadilla onto a plate, top with another plate, flip, then slide back into the pan.
Finish and rest: Cook the second side 2–3 minutes until golden and the cheese is fully melted.
Remove to a cutting board and rest 1–2 minutes. This helps the filling set.
Garnish and serve: Squeeze a little lime over the top and sprinkle the remaining cilantro. Slice into wedges and serve with sour cream, extra BBQ sauce, or salsa.