01 - Heat a large skillet or griddle over medium-high heat.
02 - Melt unsalted butter in the pan, add sliced onions, and sauté frequently until golden and caramelized, about 7 to 8 minutes. Remove onions and set aside.
03 - Season both sides of the ground beef patties evenly with salt and freshly ground black pepper.
04 - Using the same skillet, cook the patties for 2 to 3 minutes per side, or until desired doneness is achieved.
05 - Place a slice of American cheese on each patty during the final minute of cooking to allow it to melt.
06 - Stack two cheese-topped patties with caramelized onions and optional pickles, mustard, and ketchup between them to form one burger stack.
07 - Using a spatula, transfer the assembled burger stacks to plates and serve immediately while hot.