01 -
In a small bowl, mix mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce. Set aside.
02 -
In a large skillet, melt 1 tablespoon of butter over medium heat. Add sliced onions and cook, stirring occasionally, until caramelized, about 15-20 minutes. Remove and set aside.
03 -
Divide ground beef into 4 equal portions and shape them into thin patties. Season both sides with salt and pepper.
04 -
In the same skillet, cook patties over medium-high heat until cooked through, about 3-4 minutes per side. Remove from skillet and set aside.
05 -
Butter one side of each slice of rye bread. Place 4 slices, butter-side down, on a clean skillet or griddle over medium heat.
06 -
On each bread slice, place a slice of Swiss cheese, a cooked patty, caramelized onions, a slice of cheddar cheese, and another slice of bread, butter-side up.
07 -
Cook assembled sandwiches until bread is golden brown and cheese is melted, about 2-3 minutes per side.
08 -
Remove from the skillet and let rest for a minute before slicing in half. Serve immediately with the secret sauce on the side or drizzled on top.