Originally Posted by jeebus

Grilled cheese sandwiches are often made with mayo as a browning/crisping agent. You may not make it that way yourself, but if you've ever ordered a grilled cheese in a restaurant, there's a good chance you did eat one with some mayo on it.

Among many recipes (and debates) online, see for example: https://www.today.com/food/martha-st...e-ever-t105748