Add 2 tablespoons butter to a medium cast iron skillet over medium-low heat. When the butter melts, add the onion and cook for about 20 minutes, or until they have caramelized and reached a deep golden brown color.
Remove the onions from the skillet and place in a small bowl. Stir in the Worcestershire sauce. Cover and set in a warm place.
Separate the ground beef into 2 equal parts and form into a rectangular shape that is larger than the bread slices to allow for shrinkage while cooking. Form the shapes to about ⅛ to ¼ inch thick. Season both sides of the patties with Original seasoning, or seasoning of your choice.
Return to the skillet, and add 2 tablespoons of butter over medium heat. When the butter has melted, add the patties and cook for about 2 to 3 minutes, mash the burgers slightly with a spatula. Flip and continue cooking about 2 minutes or until cooked through. Remove from the skillet.
Spread a generous amount of the sauce on 2 pieces of the bread. Top 1 slice with a slice of Swiss cheese, onions, pepper jack cheese, burger patty, onions and top with the remaining bread slice.
Spread a thin layer of mayonnaise on the tops of the bread and place back in the skillet over medium heat. Toast each side for about 2 to 3 minutes, mashing down slightly with a spatula. Serve immediately.