Cooking Steps

• 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. Remove from skillet.
• Shape ground beef into two thin 4-ounce patties and season with salt and pepper. Once onions are done, add the patties to the skillet over medium-high heat and cook for 3 minutes per side.
• Wipe skillet clean and add the last tablespoon of butter. Melt butter and coat one side of the bread slices with it.
• 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 a second slice of Swiss cheese and a slice of bread 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.