In the same skillet, add the diced bell pepper and onion. Cook for 3-4 minutes until softened. Add the corn and black beans if using and cook for another minute. Remove from the skillet and set aside.
Heat a bit of butter in a clean skillet over medium heat. Place the folded quesadilla in the skillet and cook for 2-3 minutes on each side until golden brown and the cheese is melted.
Serve:
Slice the quesadillas into wedges and serve with sour cream, guacamole, salsa, and lime wedges on the side.