My Grandfather made a burger with two thin patties enclosing chopped onions, shredded cheese, mustard, and a dash of Worcestershire sauce. My meatloaf recipe is an adaption.
Meat as usual. When putting in pan, use two-thirds of the meat, leave a trough in the middle for the 'stuffing'. Last third of the meat, on top, join the edges, so the cheese juice stays in the meat, instead of running down the sides. My lady uses a glaze made of equal(?) parts brown mustard and Ketchup.
I grew up on Blind Aid and Welfare in rural Louisiana, and 80-90% of the flesh we ate came from the woods and streams in the area (My Dad spent 5 years trying to teach my mother how to cook an acceptable Squirrel Jambalaya, before giving up and doing it himself. Ground Beef was somewhat of a luxury.