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1.I prefer an 85/15 blend of store bought meat for faster prep. 2. Preheat the oven to 400: Heat 1 teaspoon oil in a 10-inch non-stick skillet over med high heat until shimmering. Add onions and 1/2 teaspoon salt to skillet. Reduce heat to med low, and cook, tossing and stirring occasionally until well browned, about 15 minutes. Once onions begin to sizzle heavily and appear dry, add 1 TB water to skillet and stir. Continue cooking until water evaporates and onions start sizzling again. Repeat process, adding 1 TB of water each time until onions are meltingly soft and dark brown, 3 times total. Transfer to a small bowl and set aside.3. Make the sauce: Combine mayonnaise, ketchup, relish, sugar, and vinegar in small bowl. Stir to combine.4. Place closed buns in oven for 2 mins until slightly darkened and crisped. Heat 1/2 teaspoon oil in 12-inch non-stick skillet over med-high heat until shimmering. Open buns and add face-down to skillet. Toast until dark brown around the edges, about 1 minute total.5. Form ground beef into four 2-ounce patties, using damp hands to press each into a patty about 3/16ths of an inch thick and 4 inches wide. Season generously with salt and pepper. Add remaining 1/2 teaspoon oil to pan, swirl to coat, and heat over med-high heat until lightly smoking. Add patties and cook without moving until well browned and crusty on 1st side, about 2 1/4 minutes. While they are cooking, spread 1 TB mustard on raw side of each patty with a spoon. Meanwhile, top each bottom bun with up half of spread, 4 slices pickles, 1 slice tomato, and lettuce. Flip patties with a thin spatula so mustard side is down and continue to cook for 2 mins. Top each patty with a slice of cheese. Divide onion mixture evenly between 2 patties. Place the other 2 patties directly on top of the onions , sandwiching them between the beef. Transfer patty stacks to bottom bun. Top with top bun, and serve immediately.
