Ricotta-Parmesan Potato Pierogi, Red Onion Thyme Butter

Steps

1. Melt the butter. Set aside 20 g (about 1½ tablespoons) of it for the dough; keep the remaining 70 g for the onion butter.
2. Make the dough: in a bowl, whisk the flour with half the salt. Add the warm milk, warm water, egg and the reserved 20 g melted butter, and stir into a shaggy dough. Knead 5 minutes until smooth, cover and rest 30 minutes at room temperature.
3. While the dough rests, make the filling: cover the potato with water in a pot, bring to a boil and simmer 15 minutes until very tender. Drain well and mash smooth.
4. Stir the ricotta, parmesan, nutmeg, black pepper and the remaining half of the salt into the mashed potato. Let the filling cool.
5. Roll the dough about 1/16 inch thick on a floured surface and cut 3-inch rounds, rerolling scraps, to make about 32. Place a heaping teaspoon of filling on each, fold into half-moons and pinch the edges firmly to seal.
6. Bring a large pot of water to a boil. Cook the pierogi in batches for 3 to 4 minutes, until they float and the dough is tender; lift out with a slotted spoon.
7. Make the onion butter: in a large skillet over medium-low heat, cook the red onion with half of the remaining 70 g butter and the thyme for 15 minutes, stirring, until soft and golden. Move the onion to a bowl.
8. Add the remaining half of that butter to the skillet over medium heat and fry the boiled pierogi in batches, 2 minutes per side, until golden and crisp-edged.
9. Return the thyme onions to the skillet, toss with the pierogi and serve hot.