To add an extra layer of indulgence to your French Onion Burgers, don't forget the cheese! Gruyère or Swiss cheese work wonderfully with the caramelized onions, providing a creamy and nutty flavor. Place a slice of cheese on top of each burger patty during the last minute of cooking, allowing it to melt and ooze over the beef and onions. The combination of the rich cheese, sweet caramelized onions, and juicy burger will create a flavor explosion in every bite.