Heat a large cast iron frying pan to medium high heat. Add a small amount of oil and spread it around and place one large flour tortilla in the pan. Keep on flipping it until air pockets are formed within the tortilla.
Add the cheese the onions and the tomatoes to the tortilla. DO NOT LAYER THE INGREDIENTS THICKLY!
Reduce the heat to low and cover the pan. Once the cheese is melted, turn off the heat. Lift up one side of the quesadilla and flip it over the other side, as if you were making an omelette.