A hamburger is traditionally a hot sandwich consisting of a cooked patty of ground beef served in a round bun. The ground beef may be mixed with chopped onion and seasonings. Toppings for hamburgers include cheese, ketchup, mustard, relish, pickle slices, onion slices, tomato slices and lettuce. Hamburger patties should be cooked until they reach an internal temperature of 160°F and are gray rather than pink in the center. The term hamburger can also refer to the ground beef itself.