The mushrooms used are generally the podgrzybki. Use dried or fresh. If you can’t get podgrzykbi, use anything else.
Use sauerkraut. If the sauerkraut is too sour you can rinse it on a strainer and leave to drain excess water and juice.
The day before you plan to make the filling, soak the dried mushrooms.
The next day simmer the mushrooms until they are soft.
Chop the sauerkraut into rather small pieces. Remember that the pierogi are rather small themselves and you do not want to give the people long strings of cabbage in their pierogi.
Sautee some onions in butter just until they are clear.
Put the chopped sauerkraut and sauteed onions into a pot.
Drain the water from the mushrooms into that mixture.
Chop the mushrooms into very small pieces and add them to the mixture.
Add a bit of butter and then water to just come to the surface of the mixture.
Now you will have to simmer the mixture for an hour and a half to two hours and sometimes more. Be sure to stir it often so it does not burn.
Your goal is to soften the sauerkraut. Don’t make mush. But soften it enough that there is almost no bite to it.
As you simmer the mixture you will have to add water a few times. Towards the end of the simmering period let the water boil down so that your final mixture is quite dry. You do not want something wet or it will leak water when you put it on the pierogi dough. If you have extra water, but the filling in a strainer and let the water drain off.
The mix by its nature is rather dull. Add salt and pepper to taste. After adding, be sure to adjust the spicing so that it gives a definite taste.