Sift the all-purpose flour into a large bowl and mix in the salt. Crack in the egg, then gradually add the water while mixing. Once the dough comes together, transfer it to a floured surface and knead until smooth and elastic, about 8–10 minutes. TIP: A well-kneaded dough won't stick to your hands but will remain soft.
Cover the dough with a kitchen towel and let it rest for 30 minutes. This allows the dough to become easier to roll out and prevents it from shrinking during shaping.
Finely chop the onion and mushrooms. Heat the sunflower oil in a skillet over medium heat and sauté the onion until translucent. Add the mushrooms, season with salt and pepper, and cook until the moisture evaporates and the flavors meld together. Let the filling cool. TIP: If it releases too much liquid, drain it to prevent the dough from becoming soggy.
Roll out the rested dough to a thickness of 2 mm. Cut out circles using a glass or cookie cutter, and place a teaspoon of mushroom filling in the center of each circle.
Fold the dough in half and press the edges together with your fingertips or a fork to seal. TIP: Moisten the edge of the dough to ensure it doesn't open during cooking.
Bring a large pot of water to a boil, lightly salt it, then cook the pierogi in batches for 3–4 minutes, until they float to the surface. Cook for another 1–2 minutes, then drain.
Serve with sour cream and, optionally, fried onions. TIP: You can also pan-fry the cooked pierogi in a little oil until golden brown for a crispier texture.
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