Also known as tavern sandwich, loose meat sandwich is an American invention consisting of a hamburger bun that is split and filled with ground beef and sautéed onions. The beef is unseasoned (it's often slowly cooked in various blends of flavorings), and the sandwich is usually additionally filled with mustard, ketchup, cheese, or pickles.
In a sandwich shop, you’ll probably get a spoon with this sandwich so you can eat all of the scattered ground meat that’s fallen off. The first tavern sandwich was prepared by David Heglin in Sioux City, Iowa in 1924, while the same thing, but with the name loose meat sandwich was prepared in Muscatine, Iowa by Fred Angell in 1926.