Preheat the oven to 350°F.
Start with cold, day-old baked potatoes. Use a cheese grater to shred them into the bottom of a 13x9-inch baking pan. Season with salt and pepper to taste.
Next, in a sauté pan on medium heat. Cook the onions , mushrooms and bell pepper until they are golden brown, about 10 minutes. Use water as needed for sticking. Once done, layer the veggie mixture on top of the shredded potatoes in the baking dish.
Grate or finely dice a block of seitan and sprinkle it over the veggie mixture
Finally, top with drained sauerkraut, and bake the casserole in the preheated oven for about 40 minutes. Enjoy!