Make the dough: In a large bowl, whisk together flour and salt. In a separate bowl, whisk the egg, warm water, and sour cream. Gradually add the wet ingredients to the dry ingredients, mixing until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for 5-7 minutes until smooth and elastic. Cover with a damp cloth and let rest for 30 minutes.
Assemble the pierogi: Divide the dough into four equal portions. Working with one portion at a time, roll it out thinly on a lightly floured surface. Use a 3-inch round cutter to cut out circles. Place about 1 tablespoon of filling in the center of each circle. Fold the dough in half to create a half-moon shape, pressing the edges firmly to seal. Ensure there are no air pockets.
Boil the pierogi: Bring a large pot of salted water to a boil. Carefully add the pierogi in batches, being careful not to overcrowd the pot. Cook for 3-5 minutes, or until they float to the surface. Remove with a slotted spoon.
Pan-fry the pierogi: Melt the butter in a large skillet over medium heat. Add the sliced onions and cook until softened and lightly browned. Add the boiled pierogi to the skillet and cook for 2-3 minutes per side, until golden brown and slightly crispy.
For a richer flavor, you can add a pinch of nutmeg to the potato filling.
If you don’t have farmer’s cheese, ricotta cheese is a good substitute.
Make sure to seal the pierogi edges very well to prevent the filling from leaking out during boiling.
You can freeze uncooked pierogi on a baking sheet until firm, then transfer them to a freezer bag for later use. Cook from frozen, adding a few extra minutes to the boiling time.