1. To make Seitan: Place seitan in baking dish. Bring pickle juice, beet juice, pickling spice, garlic powder, pepper, and 1 cup water to a boil in saucepan. Reduce heat to medium-low, and simmer 15 minutes. Strain marinade, and pour over sliced seitan. Cover and cool. Chill overnight.

2. To make Reuben Sauce: Combine all ingredients in bowl.

3. To make Sandwiches: Preheat oven to broil. Set bread slices on baking sheet. Top 4 bread slices with drained Seitan and cheese. Broil 5 to 7 minutes, or until Seitan is hot and cheese is melted. Transfer Seitan-topped bread slices to serving plates, and garnish with sauerkraut. Spread remaining 4 bread slices with Reuben Sauce, and place on top of Sandwiches.