Meanwhile, spread a tablespoon each of horseradish and mustard on one side each of two pieces of bread.
Begin assembling your sandwich, start by layering ¼ cup sauerkraut on one piece of bread, add a slice of swiss cheese and ½ cup shredded corned beef, and top with the other piece of bread.
Butter one side of the sandwich and place the buttered side down in the hot pan. Press down on the sandwich to cement the layers together before buttering the top of the sandwich.
Cook for 3-4 minutes until the bottom of the sandwich is golden brown. Carefully flip the sandwich and cook the other side for an additional 3-4 minutes until golden brown.
Serve immediately.
Notes
You can substitute mayo for the horseradish and thinly sliced deli corned beef for the shredded corned beef.