Polish Pierogi with Potato and Cheese Filling

In a large bowl, combine the flour and salt. Add the egg, then gradually add the warm water while kneading with a wooden spoon or by hand. Knead the dough until smooth and elastic, about 5-7 minutes. Cover with a clean kitchen towel and let it rest at room temperature for 30 minutes.

Peel and dice the potatoes, then boil them in salted water until tender (about 15-20 minutes). Meanwhile, finely chop one onion and sauté it in 1 tbsp of butter until golden brown. Mash the cooked potatoes with a potato masher or fork. Mix in the farmer's cheese, sautéed onion, salt, and freshly ground black pepper to taste. Mix thoroughly until you have a homogeneous filling.

On a floured surface, roll out the dough thinly (about 2-3 mm thick). Use a 6-7 cm (2.5-3 inch) round cookie cutter or glass to cut circles from the dough. Place a teaspoon of filling in the center of each circle. Fold the dough in half and firmly press the edges together to seal, ensuring the filling doesn't leak out during cooking. Repeat until all dough and filling are used.

Bring a large pot of salted water to a boil. Carefully place the pierogi into the boiling water, a few at a time, to avoid overcrowding. Cook them for 2-3 minutes after they float to the surface. Meanwhile, finely chop the other onion and sauté it in 2 tbsp of butter until golden brown. Drain the cooked pierogi and serve hot, drizzled with the fried onion butter, and optionally with a dollop of sour cream.