It may surprise you to learn that Salisbury steak isn't a "real" steak. You would be better off calling it a "Salisbury hamburger patty" than anything related to a filet mignon.
Meatloaf takes a mixture of ground beef and fillers and forms it into the shape of a bread loaf. Salisbury steak takes that same mixture, then grills or browns it in a pan.
The cooked patty is covered in onions, mushrooms, and brown gravy. Although Salisbury steak may be called a "steak," it's more similar to a hamburger patty.