Piroshki with Potato (Russian Potato Pies)

Method

1. 1Pour 310 ml of warm water (about 37 °C, comfortably warm on your wrist, not hot) into a large bowl. Stir in 15 g of sugar and 7 g of dry yeast and leave for 10 minutes, until a foamy cap forms on top. The foam tells you the yeast is alive. Add 50 ml of the vegetable oil and 1 tsp of the salt, then the 500 g of flour in three additions, stirring after each. Turn the dough out and knead it for 8 minutes, until it is smooth and soft and sticks to your hand only lightly. Don't work in extra flour, because a soft dough gives soft piroshki.
2. 2Rub 1 tsp of the oil around the bowl, put the dough back in and cover it. Leave it in a warm place out of drafts for 1 hour, until it has doubled in size.
3. 3As soon as the dough is covered, peel 750 g of floury potatoes (you will have about 640 g), cut them into 4 cm chunks and put them in a 20 cm saucepan with 1 l of cold water. Bring to the boil over high heat, add 1 tsp of salt, cover and turn the heat down to medium. Cook for 20 minutes, until a knife slides into a chunk without resistance. Meanwhile, finely chop 150 g of onion (one large onion). Heat 30 ml of the oil in a 26 cm sauté pan or deep frying pan with straight sides at least 6 cm high over medium heat, and cook the onion for 8 minutes, stirring, until it is soft and golden brown at the edges.
4. 4Ladle out 50 ml of the cooking water, then drain off the rest. Mash the potatoes while they are hot until there are no lumps left. Stir in the onion with its oil, the 50 ml of cooking water, ½ tsp of salt and ½ tsp of ground black pepper. Taste and add salt if it needs it. The mash should be soft but hold its shape on a spoon, because a runny filling leaks out of the dough. Spread it in a wide dish and let it cool for about 20 minutes, until it no longer feels warm. Warm filling softens the dough, and the seam then opens in the pan. Wipe the frying pan clean with paper towel.
5. 5Knock the air out of the dough. Rub the board and your hands with the last 10 ml of oil instead of flouring them, since flour on the edges stops them from sealing. Divide the dough into 16 pieces of about 55 g and roll each into a ball. Press or roll a ball into a round about 11 cm across and 5 mm thick. Put about 51 g of filling (2 heaped tablespoons) in the middle, leaving a clean 1 cm border. Lift the edges over the filling and pinch the whole seam firmly shut, then shape the pirozhok into an oval about 10 × 6 cm and flatten it slightly. Set it seam side down on the board. Shape the rest the same way, 2 cm apart, and cover them with a clean kitchen towel.
6. 6Before you pour in the oil, put a lid that fits the pan within reach. Pour 800 ml of oil for frying into the wiped 26 cm pan. That makes a layer 1.5–2 cm deep, enough to come halfway up the piroshki. Heat it over medium heat to 170 °C, which takes about 8 minutes, while the shaped piroshki rest for 10 minutes. Without a thermometer, drop in a scrap of dough. It should sizzle at once and bubble steadily, and the oil must not smoke. Keep the piroshki and your hands dry, because water makes hot oil spit, and do not leave the stove. If the oil catches fire, cover the pan with a lid and turn off the heat. Never pour water on it.
7. 7Lay in 4 piroshki seam side down, so the seam sets first and stays shut. Fry them over medium heat for 3 minutes, until deep golden underneath, then turn them with a slotted spoon and fry for 3 minutes more. They are done when both sides are deep golden and the piroshki have puffed up and spring back when pressed. If a side goes dark brown in less than 2 minutes, the oil is too hot and the dough will stay raw inside, so turn the heat down. Fry the rest in 3 more batches, and check with a scrap of dough that the oil is hot again before each one.
8. 8Lift the piroshki onto a tray lined with paper towels and leave them for 5 minutes to drain. Serve them warm. Put any leftovers in the fridge within 2 hours.