A patty melt is a type of sandwich consisting of a hamburger patty, topped with caramelized onions and American, Cheddar or Swiss cheese, between two slices of bread (traditionally rye, though sourdough is sometimes substituted). Patty melts are sometimes prepared using marbled rye bread. In some places, especially in the U.S., a patty melt can consist of only the hamburger patty, with cheese, on a single piece of toast/bun. The hamburger is then fried with butter on a frying pan so that the cheese melts thoroughly. In the U.S. patty melts are typically served with Thousand Island dressing on the side. It is unclear when the actual patty melt was invented, but records exist of them being served as early as the 1940s. They embellish on the traditional cheeseburger, and further take the burger back to its roots by being served on bread versus a bun.