Prepare the Onions: Place the sliced onions in a small bowl and pour boiling water over them until just covered. Leave to cool.
Make the Sauce: Whisk together mayonnaise, ketchup, mustard, and 1 tablespoon of the onion water in another small bowl. Season with salt and pepper. Adjust consistency with more onion water if needed. Set aside.
Prepare the Buns: Halve two burger buns. From the other two buns, cut out a thick middle slice. Toast or grill the middle slices on both sides and the halved buns only on the cut sides. Preheat the oven to 100°C (or 80°C for fan-assisted/gas mark 2) to keep the patties warm.
Cook the Patties: Divide the ground beef into 4 loose piles and season generously with salt. Heat ¼ tablespoon oil in a heavy frying pan over high heat. Place a portion of beef in the pan, then immediately cover it with a square of parchment paper. Using a second pan or a sturdy tool, press the beef down firmly for 10 seconds, being cautious of steam. Remove the parchment and cook the patty undisturbed for about 2 minutes to achieve a perfect sear. Flip the patty, press it down lightly with a spatula, and cook for 1 more minute. Transfer the cooked patty to a tray and place it in the oven to stay warm. Cook the remaining beef portions, adding oil as needed, and repeat the process with parchment paper for each patty.
Assemble the burgers: by spreading a layer of sauce on the bottom bun before adding your patty and toppings. Top with some of the drained onions, a lettuce leaf, a slice of cheese, and a patty. Add a few gherkin slices, then place a middle toasted bun slice on top. Spread more sauce on the middle bun, add onions, lettuce, another patty, and finish with the top bun.
Serve: Pair with oven fries and extra sauce on the side, if desired.
Enjoy your towering, flavor-packed burger masterpiece! 🍔