For the sauce: Mix mayonnaise, ketchup, mustard, pickle juice, salt, pepper, paprika, and garlic powder. Set aside.
For the onions: Thinly slice the onions and cook them with butter and oil over low heat for about 20 minutes until soft and golden.
Shape the beef: Make 4 patties, season them with salt and pepper.
Cook the patties: Cook in a hot pan. While cooking, spread yellow mustard on one side. Flip and add cheddar cheese. You'll use 2 patties per burger.
Toast the buns: In the same pan with a little butter or oil.
Assemble the burger: Add the sauce on the bottom bun, then tomato, lettuce, 2 patties with melted cheddar, caramelized onions, and more sauce. Close with the top bun.
