Cook the peeled potatoes in salted water for 20 minutes, drain and mash them while still hot.

Slice 2 onions and fry them in half the butter for 10 minutes until golden. Mix them into the mash with the crumbled twaróg, salt and plenty of pepper. Leave to cool.

For the dough, mix the flour with a pinch of salt, add the lukewarm water and melted butter, then knead for 8 minutes until smooth. Leave to rest for 30 minutes under a tea towel.

Roll the dough out to 2 mm thick and cut out 7 to 8 cm rounds with a glass. Put a teaspoon of filling in the centre, fold into a half-moon and pinch the edges firmly.

Cook the pierogi in small batches in a large pan of simmering salted water. When they rise to the surface, count 2 minutes, then lift them out with a slotted spoon.

Fry the last onion, sliced, with the bacon in the rest of the butter. Serve the pierogi topped with this mixture, with sour cream and chives.