Drain the mushrooms through a sieve, rinse and dice.
Shred the sauerkraut, place in a saucepan, add some bone stock, fat, tomato paste, cover with a lid and stew for about half an hour, stirring from time to time.
Add mushroom stock to strained bone stock, bring to a boil, then add sautéed sauerkraut and vegetables, mushrooms, peppercorns, bay leaves, browned flour with some stock mixed in, salt to taste and boil for another 5 to 10 minutes.