For the dough: 400 g (3 1/4 cups) all-purpose flour
1 egg
120 g (1/2 cup) sour cream
120 ml (1/2 cup) warm water
2 tbsp melted butter
1 tsp salt
For the filling: 500 g (1 lb) floury potatoes (russet or Maris Piper), peeled
250 g (1 cup) twaróg or farmer's cheese (dry cottage cheese)
2 onions, finely chopped
3 tbsp butter
1 tsp salt
1 tsp black pepper
To serve: 3 tbsp butter, 1 onion sliced and fried golden, sour cream, chopped chives
How to make it
Make the dough (10 min, plus 30 min rest): Mix flour and salt, add the egg, sour cream, water and melted butter and knead 5 to 6 minutes until soft and smooth. Cover and rest.
Cook the potatoes (20 min): Boil in salted water until very tender, drain well and mash while hot.
Fry the onions (15 min): Cook the onions slowly in the butter over medium-low until soft and golden, 12 to 15 minutes.
Make the filling (5 min): Mix the mash, crumbled cheese, half the onions, salt and plenty of pepper. Taste; it should be well seasoned. Cool.
Roll and cut (15 min): Roll half the dough to 2 mm thick and cut 8 cm (3 in) circles.
Fill and seal (25 min): Put a heaping tablespoon of filling in each, fold into a half-moon and pinch the edge firmly shut, crimping if you like.
Boil (5 min): Boil in salted water in batches; once they float, cook 2 minutes more. Lift out with a slotted spoon.
Finish: Serve as they are with the rest of the onions and sour cream, or fry in butter until golden on both sides, then top with fried onion and chives.
Tips
Sweet pierogi: fill with blueberries or pitted cherries tossed with a spoon of sugar, and serve with sweetened sour cream.
Boiled pierogi freeze well; fry from frozen in butter.