Mix the egg and onion together in a mixing bowl to break up the egg. Work in the ground beef, mayonnaise, bacon, bread crumbs, cheese, relish, salt and pepper. Mix with your hands until evenly combined, then pack into a loaf pan.
Spread the top of the meatloaf with ketchup. Bake in the preheated oven until a meat thermometer inserted into the center registers at least 160 degrees F, about 1 hour.