Directions
Start by making the secret sauce. In a small bowl, combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce. Mix well and set aside.
In a large skillet, melt 1 tablespoon of butter over medium heat. Add the sliced onions and cook, stirring occasionally, until they are caramelized, about 15-20 minutes. Remove from the skillet and set aside.
Divide the ground beef into 4 equal portions and shape them into thin patties. Season both sides with salt and pepper.
In the same skillet, cook the patties over medium-high heat until they are cooked through, about 3-4 minutes per side. Remove from the skillet and set aside.
Butter one side of each slice of rye bread. Place 4 slices, butter-side down, on a clean skillet or griddle over medium heat.
Top each slice of bread with a slice of Swiss cheese, a cooked patty, caramelized onions, a slice of cheddar cheese, and another slice of bread, butter-side up.
Cook until the bread is golden brown and the cheese is melted, about 2-3 minutes per side.
Once done, remove from the skillet and let the patty melts rest for a minute before slicing them in half. Serve immediately with the secret sauce on the side or drizzled over the top.