Drain the sauerkraut (drained sauerkraut should weight 450g / 16 oz), combine in a medium pot with dried mushrooms, bay leaves, and allspice berries. Add water, enough to cover the sauerkraut and mushrooms. Cook, partially covered, over medium heat for about 45 minutes or until the sauerkraut is soft.
Dice the onions. Heat 2 tablespoons of oil over medium heat in a big frying pan. Cook the chopped onion with caraway for about 10 minutes, stirring from time to time, until soft and translucent. Add marjoram, cook for another minute, transfer to a plate.
If you’re using cremini mushrooms: Wash them and pat dry with paper towels. Cut into ½ cm / ¼ inch slices. Increase the heat under the pan to high, add another tablespoon of oil and chopped mushrooms. Don’t stir for the first 3-4 minutes or until the mushrooms are browned at the bottom, then stir and cook until the mushrooms release water. Cook until it evaporates, stirring from time to time, then cook until nicely browned, transfer on a plate. It will take about 12 minutes.
Drain the cooked sauerkraut with mushrooms, reserve the liquid! It can be used to enhance the flavor of many soups, including Polish Christmas Eve Beet Soup. Discard bay leaves and allspice.
Add the cooked sauerkraut with mushrooms, pan fried cremini mushrooms and sauteed onions to the food processor bowl. Pulse several times to chop finely (but not to a paste consistency). You can also chop it finely with a knife.
Season the filling with with salt and pepper, to taste.
You can also add finely chopped prunes to the filling. It’s important to chop them with a knife by hand and not process in the food processor with the rest of the filling (or it will come out too sweet).