Heat the skillet over medium heat and add the oil, distributing it evenly across the surface.
Place one tortilla in the skillet, and sprinkle 1/2 cup of shredded cheese on one half of the tortilla.
If using, add cooked chicken or vegetables, chopped onions, garlic powder, and ground cumin on top of the cheese.
Fold the tortilla in half over the cheese and fillings.
Cook for 3-4 minutes until the bottom is golden brown and the cheese begins to melt.
Carefully flip the quesadilla using the spatula and cook for another 3-4 minutes on the other side until golden brown and the cheese is fully melted.
Remove the quesadilla from the skillet and place it on a cutting board. Let it cool for a minute before cutting into wedges.
Repeat the process with the remaining tortillas and ingredients.
Serve with salsa, sour cream, or guacamole on the side.