Pierogi Ruskie (Potato and Cheese Dumplings)

Method

1. For the dough, combine flour, warm water, egg, 1 tbsp sunflower oil, and 1/2 tsp salt in a bowl and knead into a smooth, elastic dough. Cover the dough and let it rest for at least 30 minutes.
2. For the filling, peel and quarter the potatoes, then boil them in salted water until soft. Drain and press through a potato ricer or mash finely.
3. Finely dice the half onion and fry until golden brown in 1 tbsp sunflower oil. Add to the mashed potatoes.
4. Add quark, salt, and pepper to the potato and onion mixture and mix well. Let the filling cool down.
5. On a floured surface, roll out the dough thinly. Cut out circles of about 6-7 cm (2.5-3 inches) in diameter using a glass or cookie cutter.
6. Place a teaspoon of filling in the center of each circle. Firmly press the edges of the dough together to form half-moons, pushing out any air.
7. Bring a large pot of generously salted water to a boil. Add the pierogi in batches and cook until they float to the surface. Continue to cook for another 2-3 minutes, then remove with a slotted spoon and drain.
8. Meanwhile, peel, halve, and thinly slice the whole onion. Melt the butter in a pan and fry the onion slices until golden brown.
9. Serve the cooked pierogi with the fried onions and a dollop of sour cream.