1. In a mixing bowl, combine ground beef with salt and pepper. Form into 4 patties.
2. In a skillet over medium heat, melt 1 tablespoon of butter. Add sliced onions and cook until caramelized. Remove and set aside.
3. In the same skillet, cook the patties until desired doneness. Remove and set aside.
4. In a small bowl, mix together mayonnaise, ketchup, Dijon mustard, and pickle relish to make the special sauce.
5. Spread butter on one side of each slice of rye bread. Place bread, butter side down, in the skillet. Top with a slice of Swiss cheese, a patty, caramelized onions, and a dollop of special sauce. Top with another slice of bread, butter side up.
6. Cook until bread is golden brown and cheese is melted, flipping once.