Melt the butter in a 12-inch skillet over medium heat. Add the onions and toss them in the butter. Cut a piece of parchment paper the same size of the inside of the skillet. Place the parchment paper on top of the onions as they cook to partially trap the moisture. Cook over low heat for about 20 to 25 minutes, stirring occasionally to pick up any brown bits on the bottom of the pan. When the onions have browned and caramelized, remove them from the pan and set aside.

While the onions cook, season the ground beef with the Worcestershire sauce, salt and freshly ground black pepper. Divide the meat into 4 portions and gently shape each portion into a patty about the same size as your bread.

Heat a griddle (or large skillet) to medium-high heat. Cook the burger patties for about 5 minutes on each side and then transfer them to a plate.

Assemble the patty melts by spreading some butter on one side of 8 slices of bread. Flip the slices over and spread some Best Burger Sauce on the bread slices. Top four of the slices of bread with a slice of the cheese and place a burger patty on top of the cheese. Top the patties with onions, the remaining slices of Swiss cheese and finish with the remaining bread slices, sauce side down.

Wipe down the griddle or skillet and heat to medium low heat. Grill the patty melts for about 5 minutes on each side until golden brown and the cheese has melted.

Let the burgers rest for a few minutes before serving. Cut the patty melts in half and serve them hot and juicy. Be sure to grab some napkins too!