COOK IT.
Heat the vegetable oil in a large skillet over medium-low heat. Add onions and a 3/4 teaspoon of salt, cover and cook until golden and soft, which should take about 30 minutes. Add ½ a cup of water to the skillet, scraping up the browned bits from the bottom. Simmer until the water evaporates, about 2 more minutes; transfer to a bowl.
Mix the mayonnaise, relish ketchup, and vinegar in a bowl. Squash the beef into patties, about 4 inches wide and a 1/2 inch thick.
Heat your Daytona Griddle over medium heat; brush lightly with vegetable oil. Then toast the buns on your griddle. Spread each bun bottom with 1 tablespoon of the mayonnaise mix, then top with pickles, lettuce, 2 slices of tomato and some more of the mayonnaise mix.
Season the patties with salt and pepper. Place the patties on the griddle and cook for approximately 3 minutes. Spread 1 and a half teaspoons of mustard on the side of each patty that is uncooked, flip and top with 1 slice of cheese; continue cooking about 2 more minutes till medium rare. Top the patties with caramelized onions, cheese-side up. Sandwich the double patties on the buns.