Prep the filling: In a bowl, toss shredded chicken with BBQ sauce until well coated.
You want it saucy but not dripping. Taste and adjust with salt and pepper.
Cook the bacon: Crisp the bacon in a skillet, then drain and crumble. If using pre-cooked bacon, warm it briefly to bring back the crunch.
Heat the skillet: Set a large skillet over medium heat.
Add a small pat of butter or a drizzle of oil and swirl to coat.
Build the quesadilla: Lay a tortilla in the skillet. Sprinkle a thin, even layer of cheese over half the tortilla. Top with BBQ chicken, bacon, a few red onion slices, and a light drizzle of ranch.
Add a little cilantro if you like. Finish with another light sprinkle of cheese on top to “glue” it together.
Fold and cook: Fold the tortilla over to close. Press gently with a spatula.
Cook 2–3 minutes per side until the tortilla is golden and the cheese is melted. Adjust heat if it browns too fast.
Repeat: Transfer to a cutting board and let it sit 1 minute before slicing. Repeat with remaining tortillas and filling, adding more butter or oil as needed.
Serve: Slice into wedges.
Serve hot with extra ranch, a little extra BBQ sauce, or even a squeeze of lime for brightness.