1. Prepare the secret sauce by combining all the sauce ingredients in a small bowl. Mix well and set aside.
2. Divide the ground beef into 4 equal portions and shape them into patties. Season each patty with salt and pepper.
3. Heat a skillet over medium heat and add 1 tablespoon of butter. Cook the patties for about 3-4 minutes per side, or until they reach your desired level of doneness. Remove the patties from the skillet and set aside.
4. In the same skillet, add the remaining butter and sauté the sliced onions until they are caramelized and golden brown, about 10-12 minutes. Remove from heat and set aside.
5. Assemble the patty melts by spreading a generous amount of the secret sauce on one side of each slice of rye bread. Place a slice of Swiss cheese on the non-sauced side of 4 slices of bread.
6. Top each slice of bread with a cooked patty and a spoonful of caramelized onions. Place another slice of Swiss cheese on top of the onions, followed by the remaining slices of bread, sauce side down.
7. Heat a clean skillet over medium heat and add a little butter. Carefully transfer the assembled sandwiches to the skillet and cook for 2-3 minutes per side, or until the bread is golden brown and the cheese is melted.
8. Once cooked, remove the patty melts from the skillet and let them cool for a minute before serving. Optionally, garnish with fresh parsley for an extra touch of flavor.

conclusion

Enjoy your Legal Eagle Patty Melts with Secret Sauce hot off the skillet, paired with your favorite side dish or enjoyed all on their own!