The video below shows how to make mushroom filling which is normally used for making uszka, the small Polish pierogi that is served with the borscht at Christmas time.
The filling is made up of mushrooms and onion. You can spice it with salt and pepper to taste.
I am using one small onion and 2 cups of chopped fresh mushrooms. You can use dried mushrooms that you soak overnight if you wish.
You do not have to be concerned about the type mushroom. Though many people recommend this mushroom or that, people in Poland use whatever is available.
Because the uszka are very small, the onion and mushrooms must be finely chopped so you can properly fill the uszka.
Sauté the onion in butter until it is golden. Then add the mushrooms, mix and sauté until the mushrooms are soft.
When you sauté mushrooms, they release liquids. Be sure to strain your filling to remove these liquids.
If you do not, when you fill the uszka, these liquids will run across your dough and you will have difficulty in making a good seal.
A 2 cup mix of this filling will make about 75 uszka.