Preheat the oven to 350 degrees F.
In a stand mixer, mix the beef, cracker crumbs, egg, ketchup, mustard, salt, pepper, and onion until just blended. Shape the mixture into two loaves and place side by side crosswise in a 9-by-13-by-2-inch pan. Bake the loaves for 1 hour, or until they are browned. Transfer to a platter immediately and allow the loaves to cool slightly and firm up before slicing.
Recipe adapted from Georgia Cooking in an Oklahoma Kitchen by Trisha Yearwood (c) Clarkson Potter 2008