In a skillet, melt the butter over medium heat and add the sliced onions. Cook, stirring occasionally until they are caramelized and golden brown, about 20-25 minutes.

While the onions are cooking, form the ground beef into four equal patties and season with salt and pepper.

Grill or pan-fry the patties over medium-high heat for about 4-5 minutes on each side, or until they are cooked to your desired doneness.

During the last minute of cooking, place a slice of Swiss cheese on each burger to melt.

Toast the hamburger buns on the grill or in a toaster.

To assemble, place the cooked burger on the bottom half of the bun, top with caramelized onions, and close with the top half of the bun.