In a small bowl, mix all ingredients for sauce together. Cover and refrigerate until ready to use.
Add ground chuck to a large mixing bowl. Season ground chuck with all purpose seasoning and Worcestershire sauce. Using your hands or a large spoon, mix all contents together thoroughly. Form 4 even sized balls and set aside.
Heat a large skillet over medium heat. Add 2 tablespoons of butter to the skillet. Once the butter is melted, add the onions. Season generously with salt and pepper. Continue to cook for about 10 - 15 minutes, or until onions are caramelized and tender. Remove from skillet, set aside.
In the same skillet, add 2 tablespoons of butter. Once melted, add two of the beef balls to the skillet. Take a spatula, and press until the balls are flattened. Cook for 3 minutes on each side, or until you have an internal temp of 160 degrees. Repeat this step with the other beef balls.
Reduce heat to medium-low. Scrape and wipe away any black residue from skillet. Add 1 tablespoon of butter. Once melted, add two bread slices to skillet. On one side, add 2 slices of swiss cheese, and 2 slices of cheddar cheese to the other. On one side, add the cook onions, a burger patty and a generous amount of the special sauce. Place other slice of bread with cheese, on top of slice with onions, patty and sauce. Cook sandwiches for 3 minutes on each side, or until golden brown and cheese is melted.
Slice patty melts in half, and serve warm.
