Ingredients
1sweet yellow onion,sliced thin
2tbspbutter,divided
1tbspapple cider vinegar
8ozground beef
¼tspsalt
¼tspblack pepper
4sandwich bread slices
4swiss cheese slices
mustard,optional
Preparation
In a large cast-iron skillet set over medium to medium heat, add 1 tablespoon of butter and sliced onions. Cook the onions, stirring occasionally, for about 25 minutes until they are a deep brown color.
Near the end of cooking or in about 5 minutes, add the apple cider vinegar and stir to combine. Remove from the skillet.
Shape the ground beef into two thin 4-ounce patties that are roughly the shape of the bread. Season the patties with salt and pepper.
When the onions are done in the skillet, set the heat to medium-high and add the patties. Cook for 3 minutes per side. Remove patties when done.
Wipe the 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 the 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. Enjoy!
