1. In a skillet, cook the onions over medium heat until caramelized. Remove and set aside.
2. In the same skillet, cook the ground beef, forming into 4 patties. Cook until desired doneness.
3. Mix all the special sauce ingredients in a bowl.
4. Butter one side of each slice of rye bread. Place 4 slices, buttered side down, on the skillet.
5. Add a slice of Swiss cheese, a beef patty, caramelized onions, and some special sauce on each slice of bread. Top with another slice of bread, buttered side up.
6. Cook until the bread is golden brown and the cheese is melted, flipping once.