Form Patties: Shape the ground beef into four thin patties, about ½-inch thick, and season both sides with salt and black pepper.

Caramelize Onions: Melt 2 tablespoons of unsalted butter in a skillet over medium heat. Cook sliced onions until golden brown and soft, about 15 to 20 minutes.

Cook Patties: In the same skillet, increase heat to medium-high. Cook patties for about 3 to 4 minutes on each side until desired doneness.

Butter Bread: Spread the remaining 2 tablespoons of butter on one side of each slice of rye bread.

Assemble Sandwich: Place one slice of rye bread in the skillet, layer with Swiss cheese, a beef patty, caramelized onions, another slice of Swiss cheese, and top with another slice of bread.

Grill Sandwich: Cook the sandwich for about 3 minutes on each side until golden brown and cheese is melted.

Rest & Serve: Transfer the sandwich to a cutting board, allow to rest for 5 minutes, slice and serve with optional condiments.