Cut the soy block into small cubes. Peel the onions and cut them into thin slices or fine cubes.
Heat oil in a large pan over medium-high heat. Add the soy cubes and onions. Fry for 8-10 minutes, stirring occasionally, until the soy is browned. Season well with salt and pepper.
While the soy and onions fry, heat the sauerkraut in a separate pot over medium heat.
When the soy cubes are browned, add the mushrooms to the pan. Cook for 2-3 minutes.
Add the soy, mushroom, and onion mixture to the hot sauerkraut. Stir in the tomato paste and mix well.
Let the bigos steep for 15 minutes before serving.