Sweet Potato Spinach Pierogi with Golden Onion Butter

Steps

1. Make the dough: in a bowl, mix the flour with one fifth of the salt. Add the sour cream, the egg and 60 g of the water; stir, then knead 5 minutes until smooth. Cover and rest 30 minutes at room temperature.
2. Meanwhile, put the sweet potato in a large pot with the remaining 2000 g of water and two fifths of the salt. Simmer 12–15 minutes until very tender. Lift out the sweet potato with a slotted spoon and drain well; keep the water simmering in the pot for the pierogi.
3. Melt one third of the butter in a large skillet over medium heat. Cook the garlic 30 seconds, add the spinach and cook until wilted and dry, about 3 minutes. Chop finely.
4. Mash the sweet potato, then stir in the spinach mixture, the cheddar, the parmesan, the nutmeg, the black pepper and the remaining two fifths of the salt. Cool the filling 10 minutes.
5. Roll the dough on a floured counter to about 1/8 inch thick and cut 3-inch rounds, rerolling scraps; aim for about 28. Place a heaping teaspoon of filling on each, fold into half-moons and pinch the edges firmly to seal.
6. Bring the pot of water back to a gentle boil. Cook the pierogi in batches for 3–4 minutes, until they float and the dough is tender, which cooks the egg in the dough through. Lift out with a slotted spoon.
7. Wipe the skillet, melt the remaining two thirds of the butter over medium-low heat and cook the sliced yellow onion 15 minutes, stirring, until soft and golden. Push the yellow onion aside, add the pierogi and fry 2 minutes per side until lightly crisp.
8. Serve the pierogi topped with the yellow onion butter and sprinkled with the green onions.