In a skillet over medium heat, melt 1 tbsp of butter and sauté the onions until caramelized, about 10 minutes. Set aside.
Season the ground beef with salt, pepper, and Worcestershire sauce. Shape into two large patties.
In the same skillet, cook the patties over medium heat for about 3-4 minutes per side until cooked through.
Spread mayonnaise, mustard, and ketchup on the rye bread slices.
Assemble the sandwich by placing a slice of Swiss cheese, a beef patty, caramelized onions, and another slice of cheese between two bread slices.
Melt the remaining butter in the skillet and cook the sandwiches on both sides until golden brown and crispy, about 3 minutes per side.
Slice in half and serve hot.