Directions

1. 1

   Combine the flour and 1 teaspoon kosher salt in a large mixing bowl. In a separate small bowl, whisk together the sour cream, vegetable oil, cold water, and egg until well blended and smooth. Pour the wet mixture into the flour mixture and use a fork to stir until shaggy pieces form. Switch to using your hands and knead the mixture in the bowl until it comes together as a rough mass. Transfer to a lightly floured surface and knead by hand for 3 to 4 minutes until the dough becomes soft and smooth; avoid adding excess flour as this will toughen the dough. Wrap the dough tightly in plastic wrap and let it rest at room temperature for 30 minutes.

2. 2

   While the dough rests, heat a large skillet over medium heat. Add 4 tablespoons of the unsalted butter and let it melt. Add the chopped onions and season with 1/2 teaspoon kosher salt. Cook, stirring occasionally, until the onions turn deep golden brown and become very soft and tender, approximately 15 to 20 minutes. Transfer 1/3 cup of the caramelized onions to a small dish and set aside. Leave the remaining onions in the skillet and set the skillet aside for later use.

3. 3

   Place the potato chunks in a large saucepan and cover with salted water by about 1 inch. Bring to a gentle simmer and cook until the potatoes are completely tender and easily pierced with a fork, 15 to 18 minutes. Drain the potatoes thoroughly in a colander and return them to the pot. Add the grated Cheddar cheese, 1/4 cup sour cream, the remaining 4 tablespoons unsalted butter, 1 teaspoon kosher salt, and the reserved 1/3 cup of caramelized onions. Mash everything together with a potato masher until the mixture is smooth and creamy. Transfer the filling to a bowl and allow it to cool to room temperature.

4. 4

   Once the filling has cooled, cut the rested dough into two equal halves. Fill a small bowl with water. Dust your work surface lightly with flour and roll one dough half into a very thin 15-inch square (it does not need to be perfectly square; the key is rolling it thin enough that you can see through to the counter). Using a 3 1/2-inch round cutter, cut out as many dough circles as possible. Place about 1 tablespoon of filling in the center of each circle. Dip your finger in water and lightly brush around the rim of the dough circle, then fold it in half to create a half-moon shape. Press and seal the curved edges firmly to ensure they are completely closed. Arrange the sealed pierogi on a floured baking sheet. Repeat the rolling, filling, and shaping process with the second dough half. Combine any leftover dough scraps, roll them out, and shape additional pierogi; these will be slightly thicker but still delicious. The pierogi are ready to cook immediately or can be frozen for future use.

5. 5

   Fill a large pot with salted water and bring it to a rolling boil. Return the skillet with the remaining caramelized onions to low heat and add the final 2 tablespoons of unsalted butter, stirring until melted. Working in batches, add approximately half of the pierogi to the boiling water. Gently stir them so they don't stick. Once they float to the surface, cook for an additional 3 to 4 minutes until they are tender. Remove each batch with a spider strainer or slotted spoon and transfer directly to the butter and onion mixture in the skillet. Repeat with the remaining pierogi. Once all the pierogi are in the skillet, increase the heat slightly and gently toss them to coat evenly with the butter and onions. Taste and adjust seasoning with salt as needed. Serve hot with sour cream on the side.