In a large mixing bowl, combine the egg, sugar, and salt until smooth. Slowly mix in warm milk and vegetable oil. Gradually add sifted flour and instant yeast. Knead for 8-10 minutes until smooth and elastic.

Cover the dough with a clean towel and let it rise in a warm area for about 60-90 minutes or until doubled in size.

While the dough is rising, peel and chop the potatoes. Boil them in salted water for 10-25 minutes until fork-tender. Drain and mash.

Melt unsalted butter in a skillet. Add chopped onion and sauté for 5 minutes. Stir in ground meat, cooking until browned. Season with salt, pepper, and optional paprika.

Combine cooled mashed potatoes with meat mixture, adding fresh dill and season to taste. Mix well and set aside.

Once the dough has doubled, punch it down and divide into 12 pieces. Roll each piece into an oval and fill with a tablespoon of filling. Pinch edges to seal.

Heat oil in a deep pot to 350-375°F. Fry piroshki in batches until golden brown, about 5 minutes, flipping halfway through.

Cool slightly on paper towels, then serve warm with sour cream.