Cream Cheese Mushroom Pierogi with Onion Butter

Steps

1. Make the dough first: whisk the flour with half the salt in a bowl. Add the sour cream, egg and water and stir into a shaggy dough.
2. Knead the dough on a lightly floured counter for 5 minutes until smooth, cover with the bowl and rest 30 minutes while you make the filling.
3. Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms and cook 8–10 minutes until their liquid evaporates and they brown.
4. Stir the garlic and thyme into the mushrooms for 30 seconds, then move them to a bowl and let cool 10 minutes. Wipe out the skillet.
5. Mix the cream cheese, mozzarella, sliced green onions, black pepper and the remaining half of the salt into the cooled mushrooms to make the filling.
6. Roll the dough to about 1/16 inch thick and cut 3-inch rounds, rerolling scraps, for about 32 rounds.
7. Place a heaping teaspoon of filling on each round, fold into a half-moon and pinch the edges firmly to seal.
8. Bring a large pot of water to a boil. Boil the pierogi in batches until they float, then 2 minutes more, and lift them out with a slotted spoon.
9. Melt half the butter in the skillet over medium-low heat. Add the sliced yellow onion and cook 15 minutes, stirring, until soft and golden; move to a plate.
10. Melt the remaining half of the butter in the skillet over medium heat and fry the boiled pierogi 2 minutes per side until golden in spots.
11. Return the golden yellow onion to the skillet, toss gently with the pierogi and serve hot.