Potato-Cheddar Pierogi with Dill Yogurt

Steps

1. Make the dough: in a bowl mix the flour with half of the salt. Add the egg, the sour cream and the lukewarm water and stir into a shaggy dough. Knead 5 minutes until smooth, cover and rest 30 minutes.
2. Meanwhile make the filling: put the potato in a large pot, cover with fresh cold tap water (not the measured water), bring to a boil and simmer 15 minutes until very tender. Drain, then return to the pot.
3. In a large skillet, melt one-third of the butter over medium heat and cook the onion with a pinch of the remaining salt for 15 minutes, stirring, until deep golden. Set the onion aside.
4. Mash the potato with the cheddar, the black pepper, the remaining salt and one-quarter of the cooked onion until smooth. Let cool.
5. Make the herb yogurt: stir the Greek yogurt with the dill, the garlic and the lemon juice. Chill until serving.
6. Roll the dough about 1/16 inch thick on a floured counter and cut 3-inch rounds, rerolling scraps, for about 32. Put a spoonful of the potato-cheese filling on each, fold into half-moons and pinch the edges firmly to seal.
7. Bring a large pot of fresh tap water to a boil. Cook the pierogi in batches about 3 minutes after they float, then lift out with a slotted spoon.
8. Wipe the skillet, melt the remaining two-thirds of the butter over medium-high heat and fry the boiled pierogi in batches 2 to 3 minutes per side until golden and crisp. Add the remaining cooked onion to warm through.
9. Serve the pierogi topped with the onions, with the dill yogurt on the side.