In a large skillet over medium-low heat, melt 2 tablespoons of butter. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions turn a deep golden brown, about 20-25 minutes. If they start to stick, add a tiny splash of water to deglaze the pan. Set aside once caramelized.

In a mixing bowl, combine the ground beef, Worcestershire sauce, garlic powder, salt, and pepper. Form the mixture into oval patties that are slightly larger than the bread slices.

Increase the heat to medium-high. Cook the patties for about 3-4 minutes per side until they have a beautiful crust. Remove the patties and wipe the excess grease from the skillet, leaving a little for flavor.

Spread butter on one side of each bread slice. Place two slices of bread, buttered side down, in the skillet. Layer a slice of cheese, the cooked beef patty, a mountain of caramelized onions, and another slice of cheese. Top with the second slice of bread, buttered side up.

Cook over medium heat for 2-3 minutes until the bottom is golden brown. Carefully flip the sandwich. If you have a heavy lid or another skillet, use it to press down lightly to ensure the cheese is fully melted. Cook until the second side is crispy and the cheese has completely melted.

Let the sandwich sit for a minute before slicing it on a diagonal. Serve with ketchup and mustard on the side.