Half the roll and assemble the sandwich with the listed ingredients. Melt about 1 tbsp of butter in a heavy skillet over low heat. Place the sandwich in the skillet while pressing down with a spatula or another skillet. Heat about 30 seconds on each side until the cheese has softened but not quite melted. This can also be heated over a backyard barbecue.
Do not confuse this with the quesadilla, that's totally different. This is usually made with Mexican string cheese known as Oaxacan. It's similar in texture and color to mozzarella, but tastes a bit saltier. If you can't find it, use any string cheese.