In a large cast-iron skillet set over medium heat, add 1 tablespoon of butter and sliced onions. Cook onions, stirring occasionally, for about 25 minutes until they are a deep brown color. Add apple cider vinegar near the end of cooking and stir to combine, then remove from the skillet.

Shape ground beef into two thin 4-ounce patties that are roughly the shape of your bread. Season patties with salt and pepper. When onions are done, set the heat to medium-high and add the patties. Cook for 3 minutes per side. Remove patties when done.

Wipe your skillet clean and add the last tablespoon of butter. Once melted, add all the bread slices to coat one side with butter. Remove two slices, then build your sandwich on the two slices remaining in the skillet.

On each slice of bread in the skillet, add a slice of Swiss cheese, a patty, half of the caramelized onions, and top with the second slice of Swiss cheese. Top each sandwich with the slices of bread removed earlier, buttered side up.

Over medium-low heat, cook the sandwiches for 3 to 4 minutes per side until the bread is toasted and the cheese is melted.

Slice sandwiches in half and serve immediately while warm.