In a large mixing bowl, combine the all-purpose flour, salt, sugar, and dry yeast.

Add warm milk and warm water to the dry ingredients and mix until a dough starts to form.

Incorporate vegetable oil into the dough and knead until smooth and elastic, about 5-7 minutes.

Cover the dough with a cloth and let it rise in a warm place for about 1 hour or until doubled in size.

Peel and boil the potatoes in salted water until tender, about 20 minutes. Drain and mash them.

In a pan, heat vegetable oil and sauté finely chopped onion until golden and soft, about 5 minutes.

Mix the mashed potatoes with the sautéed onion. Season with salt, pepper, and optional dill to taste.

After the dough has risen, punch it down and divide it into 17 equal portions.

Roll each portion into a small circle, about 4-5 inches in diameter.

Place a spoonful of potato filling in the center of each dough circle. Fold the dough over the filling and pinch the edges to seal.

Heat vegetable oil in a deep frying pan or skillet over medium heat.

Fry the piroshki in batches, turning occasionally, until golden brown on all sides, about 3-4 minutes per batch.

Remove the piroshki from the oil and drain on paper towels to remove excess oil.