Patty melt

A patty melt is an American grilled hamburger consisting of a ground beef patty topped with melted cheese and caramelized onions between two slices of griddled seeded-rye bread. The patty melt is variously described as a variant of the traditional American cheeseburger, topped with fried onions and served on sliced bread instead of a traditional bun, or as a variant of a grilled cheese that includes a hamburger patty and fried onions.

Source: Wikipedia