Heat the frying pan to a medium heat. Add some butter or oil and spread evenly.
Take a large tortilla, place it flat on the pan and wait until air pockets for
Flip the tortilla and add the cheese all over it (and other ingredients if your are using them)
Lower the heat and cover the pan.
When you see the cheese has melted use a spatula to lift one side of the tortilla and fold it in half as if you are making an omelette. It should be browned (but not burned) on both sides. If it is not browned then turn the heat up and flip the quesadilla a few times.
Remove the quesadilla from the pan and use a pizza cutter to cut into 4 wedges. Serve with some salsa, sour cream, guacamole and lettuce and top with some fresh cilantro.
Notes
to make it gluten-free use a gluten-free tortilla
to make it vegan use meltable vegan cheese
use toppings such as chilis, dried chili flakes, cooked chicken, cooked ground beef, cooked pork, avocados, mushrooms, eggs, onions, prawns