8.

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 medium-high heat until lightly smoking. Add burger patties and cook without moving until well browned and crusty on first side, about 2 1/2 minutes. While they are cooking, spread 1 tablespoon 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 1 minute. Top each patty with a slice of cheese. Divide onion mixture evenly between two patties. Place the other two 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.