01 -
In a small bowl, combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce. Mix thoroughly until smooth and set aside.
02 -
Melt 15 g butter in a large skillet over medium heat. Add sliced onions and cook, stirring occasionally, until deeply golden and caramelised, about 15 to 20 minutes. Remove onions from the skillet and set aside.
03 -
Divide ground beef into 4 equal portions and shape each into a thin patty. Season both sides generously with salt and black pepper.
04 -
In the same skillet over medium-high heat, cook the patties for 3 to 4 minutes on each side until cooked through. Remove from the skillet and set aside.
05 -
Butter one side of each rye bread slice. Place 4 slices, buttered side down, on a clean skillet or griddle over medium heat. Top each with a slice of Swiss cheese, a cooked beef patty, caramelised onions, a slice of cheddar cheese, and another slice of bread, buttered side up.
06 -
Cook each sandwich until bread is golden and cheese is melted, about 2 to 3 minutes per side. Remove from heat, allow to rest 1 minute, then slice in half. Serve immediately with the secret sauce on the side or drizzled over the sandwich.