In a large skillet under medium heat, melt 2 tablespoons of butter and add in the onions. Sauté the onions starts until it starts to soften. Mix in the Worcestershire sauce and salt and black pepper to taste. Continue to cook the onions while stirring until fully caramelized, about 8-10 minutes or longer as needed. Spoon the caramelized onions into a dish and set aside to cool.

Divide the ground beef into (4) 1/4 pound portions and shape into a ball.

In the same skillet used to make the caramelized onions, drizzle in some oil and increase the heat to medium high.

In batches, add the portions of beef into the skillet. Using a piece of parchment paper and a spatula, flatten the beef into a thin patty. Season with salt and black pepper and cook for 3-4 minutes. Flip the patties, season with salt and black pepper and cook for an additional 2-3 minutes or until preferred doneness.

Add (2) slices of American cheese over each patty. Cover the skillet with a lid to allow the cheese to melt slightly. Stack (2) patties on top of each other to form a double patty.

Place down a slice of bread, spread over a layer of burger sauce (about 1 1/2 to 2 tablespoons), top with a slice of mozzarella or Swiss cheese and place down (1) set of the double patties.

Spoon over a layer of caramelized onions, drizzle over some burger sauce and top with a slice of mozzarella or Swiss cheese. Close sandwich with a slice of bread.

Repeat with remaining ingredients for a total of (2) double patty melt sandwiches.

In a skillet over medium heat, melt the remaining butter. Add the assembled sandwiches into the skillet and toast until golden brown, about 2-3 minutes per side.

Serve patty melts immediately with a side of fries or chips and extra burger sauce for dipping.