In a small bowl, toss the pulled pork with BBQ sauce to coat it evenly.
Assemble the quesadilla
Lay one tortilla flat on a plate or cutting board. Sprinkle half of the cheese evenly over it. Spread the BBQ pork on top, followed by the red onion and cilantro (if using). Add the remaining cheese and top with the second tortilla.
Cook the quesadilla
Heat a skillet over medium heat and add butter or oil. Once hot, place the assembled quesadilla in the pan. Cook for about 3–4 minutes, pressing down gently with a spatula.
Flip carefully
When the bottom is golden brown, carefully flip the quesadilla using a large spatula. Cook for another 3–4 minutes until the cheese is melted and the second side is crispy.
Slice and serve
Transfer to a cutting board and let it rest for a minute before slicing into wedges. Serve hot with sour cream, salsa, or extra BBQ sauce for dipping.