If frozen, thaw the mushrooms.
Chop the onions. Fry with the mushrooms until golden.
Season with pepper. Set aside.
Choose one of the pierogi dough recipes from here, or follow this basic recipe below:
Sift some flour onto your worktop. Make a well, pour in a small amount of hot water.
Knead together rapidly. Gradually add enough water for the dough for it to become soft and elastic. Avoid adding too much, it might become too sticky to handle.
Divide the dough into parts. Sprinkle the worktop with flour, spread one piece of dough on top. Roll into a thin layer.
Use a glass to cut into circles. Place a spoonful of mushroom filling in the middle of each circle.
Fold dough over filling. Press the edges firmly together.
Continue until you're out of all ingredients.
Boil a pot of salted water, reduce the heat to a simmer.
Drop a few pierogi into the pot. Cook for around 5 to 6 minutes, until they start to float.
Collect cooked dumplings with a slotted spoon. Serve with melted butter or another topping of your choice.