Arrange 6 of the bread slices on a work surface. Top each slice with 1 slice of the Swiss cheese, red onion, chicken mixture, and the second slice of Swiss cheese. Top with the second slice of bread.
Top each toast with 1 tablespoon of sauerkraut, 1 slice of corned beef and a small slice of Swiss cheese. Place baking sheet back under the broiler and broil until cheese is bubbly and golden.