This step is for both the filling and the serving topping, so make a larger batch than you think you need. Melt 2 tablespoons of butter in a wide heavy pan over the lowest possible heat. Add the finely diced onion for the filling and the sliced onions for serving in the same pan. Cook over very low heat, stirring occasionally, for 30 to 40 minutes until completely soft, golden and sweet. This cannot be rushed. High heat produces bitter, unevenly cooked onion rather than the sweet, deeply caramelized result that makes this dish. Set aside. Reserve half for the filling and half for serving.

Bring a large pot of well-salted water to a boil. Add the peeled potato chunks and boil until completely tender, 15 to 20 minutes. Drain thoroughly and return to the dry pot over low heat for 1 to 2 minutes to drive off any remaining moisture. This step is important. Wet potato filling makes the pierogi heavy and can cause the dough to split during boiling.

Mash the potatoes until completely smooth with no lumps. Allow to cool to room temperature. Add the twaróg or drained ricotta and mix thoroughly. Add the reserved caramelized onion for the filling. Season generously with salt and white pepper. Taste and adjust. The filling should be well seasoned and slightly salty since the dough will dilute the seasoning somewhat. If the filling seems too loose to hold its shape when you pinch a tablespoon of it, it needs more draining or slightly more cheese.

Place the flour and salt in a large bowl. Add the softened butter and rub it into the flour with your fingertips until the mixture resembles very fine breadcrumbs. Add the hot water gradually, a few tablespoons at a time, mixing with a fork or your hands after each addition. Stop adding water when the dough just comes together into a smooth, pliable ball that is not sticky. You may not need all the water.

Turn the dough out onto a lightly floured surface and knead gently for 2 to 3 minutes until completely smooth and elastic. The dough should be noticeably soft and pliable, almost silky. Wrap in cling film or cover with an inverted bowl and rest for at least 30 minutes at room temperature. This resting period is essential. It relaxes the gluten and makes the dough much easier to roll thinly without it springing back.

Divide the rested dough into two pieces. Work with one piece at a time, keeping the other covered. On a lightly floured surface, roll the dough to approximately 2mm thickness, as thin as you can manage while keeping it intact. Use a round cutter approximately 8 to 9cm in diameter, or the rim of a drinking glass, to cut circles from the rolled dough. Re-roll the scraps and cut again.

Place a rounded teaspoon of filling in the centre of each dough circle. Do not overfill. The filling should leave a clear border of dough around the edge. Fold the dough over the filling to create a half-moon shape. Press the edges firmly together with your fingers to seal completely. Then go around the sealed edge and crimp it by pressing and folding small pleats with your thumb and index finger. The crimped edge is both decorative and functional, providing a stronger seal that prevents the pierogi from opening during boiling.

Place the sealed pierogi on a lightly floured surface or baking sheet while you finish making the rest.

Bring a large pot of well-salted water to a rolling boil. Working in batches of 8 to 10 to avoid crowding, lower the pierogi gently into the boiling water. They will sink initially and then rise to the surface. Once they have risen to the surface, cook for a further 2 to 3 minutes until the dough is cooked through and completely tender. Remove with a slotted spoon and transfer to a warm plate.

This is the step that most Polish home cooks add and that elevates the finished pierogi significantly. Melt a generous amount of butter in a wide pan over medium-high heat. Add the boiled pierogi in a single layer without crowding. Fry for 2 to 3 minutes per side until golden and slightly crisp on both surfaces. The contrast between the crisp exterior and the soft filling is the version most commonly served in milk bars and homes across Kraków.

Arrange on a warm plate. Spoon the remaining caramelized onion generously over the top. Drizzle with melted butter. Scatter fresh chives over everything. Serve immediately with sour cream alongside.