Preheat the oven to 450 degrees.
Coat each tortilla generously with cooking spray and place sprayed side down on a large baking sheet.
Sprinkle ½ cup of cheese evenly over each tortilla.
On one half of each tortilla, place ¼ of the roast beef and ¼ of the French fried onions.
Bake for 5 to 7 minutes or until the cheese is melted and tortilla is lightly browned.
Carefully fold the cheesy half of each tortilla over the beef and onion half, and cut each quesadilla into two or three wedges with a pizza wheel.
Garnish with your favorite toppings.