Mix your ground beef with salt and pepper in a bowl. Divide it into equal portions and shape them into thin patties slightly larger than your bread slices to allow for shrinkage during cooking.

Heat your cast iron skillet over medium-high heat. Add the beef patties, cooking each side for about 3-4 minutes or until your preferred level of doneness. Remove them from the skillet and set aside.

Reduce the heat slightly and toss in your thinly sliced onions along with some butter. Add a pinch of salt and a sprinkle of sugar to help with the caramelization. Stir frequently for about 15-20 minutes until the onions turn soft, golden brown, and sweet.

Butter one side of each bread slice. Place one slice butter-side down in your cast iron skillet. Add a layer of Swiss cheese, followed by a patty, a generous portion of caramelized onions, another slice of cheese, and finally, top it with another bread slice (butter-side up).

Cook your assembled sandwiches on medium heat until the bread is golden and crisp, and the cheese melts into gooey goodness. Flip carefully after 2-3 minutes and repeat for the other side. Cut diagonally and serve hot.