01 -
Combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce in a small bowl. Mix well and set aside.
02 -
Melt 1 tablespoon of butter in a large skillet over medium heat. Add sliced onions and cook, stirring occasionally, until caramelized, about 15-20 minutes. Remove from skillet and set aside.
03 -
Divide ground beef into 4 equal portions, shaping each into thin patties. Season both sides with salt and pepper.
04 -
In the same skillet, cook patties over medium-high heat for 3-4 minutes per side, until fully cooked. 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 -
Top each bread slice with 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 sandwiches until the bread is golden brown and the cheese is melted, about 2-3 minutes per side.
08 -
Remove from skillet, let rest for 1 minute, and slice in half. Serve immediately with the secret sauce on the side or drizzled over the top.