Heat the oil in a large skillet over medium-high heat.
Add the pineapple and a pinch of salt. Cook for 3 to 4 minutes, stirring occasionally, until lightly caramelized and golden around the edges. Transfer to a bowl.
Add the onion to the skillet and cook for 1 minute until slightly softened.
Add the al pastor pork and cook for 2 to 3 minutes until heated through and lightly crisp around the edges.
Remove the skillet from the heat and gently fold in the caramelized pineapple.
Lightly spread butter on one side of each tortilla.
Place one tortilla butter-side down in a skillet over medium heat.
Sprinkle ¼ cup of the cheese blend over one half of the tortilla.
Add about ½ cup of the pork and pineapple mixture over the cheese.
Top with another ¼ cup of the cheese blend.
Fold the tortilla over the filling.
Cook for 2 to 3 minutes per side until golden brown and crisp and the cheese is fully melted.
Repeat with the remaining tortillas and filling.
Let the quesadillas rest for 1 minute before slicing.
Garnish with cilantro and serve with salsa verde, guacamole, lime wedges, and pickled red onions if desired.