Prepare the Secret Sauce
In a small bowl, combine mayonnaise, ketchup, Dijon mustard, pickle juice, paprika, and garlic powder. Mix until smooth and set aside.

Caramelize the Onions
Heat olive oil in a skillet over medium-low heat. Add sliced onions and cook, stirring occasionally, until they are golden brown and caramelized, about 20–25 minutes. Remove from skillet and set aside.

Cook the Beef Patties
In a bowl, mix ground beef with salt, pepper, and Worcestershire sauce. Divide into four equal portions and shape into thin patties. In the same skillet over medium heat, cook patties for 3–4 minutes per side, or until desired doneness. Remove from skillet and set aside.

Assemble the Sandwiches
Butter one side of each slice of rye bread. On the non-buttered side of four slices, layer one slice of Swiss cheese, a beef patty, caramelized onions, a slice of cheddar cheese, and a generous spoonful of secret sauce. Top with the remaining bread slices, buttered side facing out.

Grill the Sandwiches
Heat a skillet or griddle over medium heat. Place sandwiches on the skillet and cook until the bread is golden brown and the cheese has melted, about 3–4 minutes per side. Press down gently with a spatula to ensure even grilling.

Serve
Remove sandwiches from skillet, slice in half if desired, and serve hot.