Ukrainian
Mix the flour with 0.5 tsp salt, add the egg, 1 tbsp oil and the water, and knead for 8-10 minutes into a smooth, firm dough. Cover and rest for 30 minutes.
Boil the potatoes in salted water for 20 minutes until soft, drain and mash them while hot.
Fry the onions in 5 tbsp oil over medium heat for 12-15 minutes until golden; mix one third of them into the mashed potatoes with 1 tsp salt and the pepper, and let the filling cool.
Roll the dough out on a floured surface to 1-2 mm and cut out 7 cm circles with a glass; knead the scraps together and roll again.
Put 1 heaped tsp of filling on each circle, fold it in half and pinch the edges firmly to seal. You get about 70 vareniki.
Cook the vareniki in batches in 3 l of boiling salted water for 4-5 minutes after they float, then lift them out with a slotted spoon.
Serve hot, topped with the remaining fried onion and its oil, with sour cream on the side.