Put the sauerkraut in a bowl
Put the mushroom in a dry skillet over high heat, cover with a lid. Once the mushrooms release the moisture, take off the lid and cook until the water evaporates.
Add in the oil and onion and saute over low heat for about 10 minutes. Once cool mix together with the sauerkraut
Roll dough out to ⅛"thickness (you may need to divide the dough in half and roll dough out at a time if your working area is small.)
Cut out circles using a cup 3" in diameter.
Place about a heaping teaspoon of filling in the centers and seal pierogi. Transfer to a floured tray and continue to work on the dough rolling out the scraps. (Tip: use a pierogi maker for quick and easy results or you can crimp with a fork.)
Freeze or boil in water for 3 minutes.
To cook frozen: Boil in water for 3-4 minutes
sauté in butter until nicely seared (about 2-3 minutes.)
Serve with sour cream and garish with some chives