In a large cast-iron skillet set over medium heat, melt 4 tablespoons of butter. Add the onion and cook for 20 minutes until caramelized, stirring occasionally. Transfer onions to a plate and set aside.
½ cup unsalted butter, 1 sweet onion
Meanwhile, combine the ground beef, Worcestershire sauce, garlic powder, salt, and pepper. Mix until combined, then divide the mixture into 4 portions and form 4 oblong patties.
1½ pounds 85% lean ground beef, 1 teaspoon Worcestershire sauce, ½ teaspoon garlic powder, 1 teaspoon kosher salt, ½ teaspoon ground black pepper
Melt another 2 tablespoons of butter in the skillet. Add the patties and cook 3-5 minutes per side, until desired doneness is reached. Transfer the cooked patties to a plate and tent with foil to keep warm.
Drain the fat from the skillet, then return it to the stovetop. Melt the remaining butter and, working in batches, toast the bread until golden, 2 minutes per side, adding more butter if necessary.
8 slices marble rye bread
Place one slice of Swiss cheese on a piece of bread, then top with a patty, caramelized onions, another slice of cheese, and a second piece of bread. Repeat to create 4 patty melts.
8 slices Swiss cheese