In a large cast-iron skillet set over medium to 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. Near the end of cooking (about 5 minutes), add apple cider vinegar 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 in your skillet, 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 to the skillet. Once the butter melts, add all the bread slices to the skillet, so one side of each slice is coated with butter. Remove two of the four pieces of bread and start building your sandwich on the two pieces remaining in the skillet.

On each slice of bread remaining 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 slice of bread removed from the skillet 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.