Start with the sauce. In a mixing bowl, add ketchup, mayo, sweet relish, apple cider vinegar and sugar. Mix the ingredients together and set aside.
To sauté the onions, preheat the griddle or grill to a medium heat and spread some olive oil on the flat top. Dice up a white onion and throw them on the griddle. Sauté for about 15 minutes until they are soft. Move them to the side and turn down the burner to a low heat to keep them warm.
Shape 3 oz. round balls of ground beef.
On a medium heat, place the beef on the griddle and smash it down using a spatula.
Season the patties with Dad's All Purpose seasoning and add some yellow mustard.
Grill 2 minutes per side. At the end, add a slice of American cheese and allow it to melt over the patties before removing off the grill.
To assemble the burger, toast up the buns, add a generous layer of the sauce, add lettuce, sliced tomato, 1 or 2 patties, a scoop of the sautéed onions and more sauce.
