Check off each step as you cook.

1. 01Rest the dough.Mix flour with half the salt, warm water, and oil. Knead for 5–7 minutes to a smooth, soft dough, adding a little water only if dry. Cover and rest for 30 minutes.
2. 02Prepare the filling.Meanwhile, boil potatoes for 15–20 minutes until tender, drain thoroughly, and mash without adding milk. Cook the onion in two-thirds of the butter over medium-low heat for 8–10 minutes. Reserve one-third of the cooked onion for serving.
3. 03Mix and cool.Combine potato with the cheese, remaining cooked onion, pepper, and remaining salt. Taste for seasoning and let cool until comfortable to handle.
4. 04Roll, fill, and seal.Roll the dough about ⅛ inch / 2–3 mm thick on a lightly floured board. Cut 3-inch / 8 cm circles, gathering scraps and rerolling gently. Add a rounded tablespoon of filling to each, fold into a crescent, and pinch closed. Aim for about 32 dumplings in the base batch.
5. 05Boil in batches.Bring a wide pot of salted water to a gentle boil. Add a few pierogi at a time, stir carefully to stop sticking, and cook for 2–3 minutes after they float. Lift out with a slotted spoon and drain.
6. 06Finish with onion butter.Warm the remaining butter and reserved onion in a skillet. Toss the cooked pierogi gently in it, or pan-fry for 2–3 minutes per side if you prefer a little color. Serve with optional sour cream.

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