Place dried mushrooms in a pot of water and let it stand, preferably overnight
The day of making pierogi, cook porcini mushrooms in the water that they soaked in
Rinse the sauerkraut and place in another pot, cover with water and cook for about an hour
In the meantime heat up 1 tablespoon of oil, add chopped onions and cook until soft, about 5 minutes
Add sliced fresh mushrooms to the onions and cook until soft, about 15 minutes
When dried mushrooms are cooked, remove them from the water with slotted spoon, reserve the water
Rinse them well under running water to make sure that they are perfectly clean
Once all your ingredients are cooked, you can add sauerkraut, fresh mushrooms and dried mushrooms to the food processor and process until finely chopped but not mushy
Placed processed filling back to the pan and using strainer add 1/4 to 1/2 cup of dried mushrooms water and cook for a few minutes
Check for seasoning, add salt and pepper and cook for few more minutes until all the ingredients are merged.
Let it cool before starting to assemble pierogi.