A patty melt is a type of sandwich consisting of a hamburger patty, sometimes topped with caramelized onions and Swiss cheese between two slices of bread Patty melts are sometimes prepared using marbled rye bread. In some places, especially in the U.S., a patty melt might consist only of the hamburger patty on a single piece of toast or a single bun. The hamburger is then fried with butter on a frying pan so that the cheese melts thoroughly. It is unclear when the patty melt was actually invented, but records exist of them having been served as early as the 1940s. The patty melt is a variant of the traditional American cheeseburger, taking the sandwich back to its roots by serving it on bread versus a bun.