Make the Secret Sauce
In a small bowl, combine the mayonnaise, ketchup, mustard, pickle juice, paprika, and garlic powder.
Whisk until fully blended and set aside.
Cook the Onion and Beef
Heat the olive oil in a skillet over medium-low heat. Add the sliced onions. Cook for 30-40 minutes, stirring occasionally, until the onions are deeply golden brown and caramelized. If the onions start to stick, add a splash of water.
In a bowl, mix together the ground beef, salt, pepper, and Worcestershire sauce. Form into four thin, oval-shaped patties.
Cook the patties in the same skillet over medium heat until browned, about 2-3 minutes per side. Remove and set aside.
Assemble and Cook the Sandwiches
Spread one side of each bread slice with softened butter.
Layer the bottom bread slices buttered-side down with Swiss cheese, beef patty, caramelized onions, secret sauce, cheddar cheese, and top bread slice buttered-side up.
Cook in batches in the skillet over medium heat until both sides are golden brown and the cheese is fully melted, about 1-2 minutes per side.
Storage
Store leftover secret sauce covered in the fridge for up to 5 days.
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