In a large mixing bowl, whisk together flour and salt. Create a well in the center and add the egg, sour cream, melted butter, and warm water.
Using a wooden spoon, gradually incorporate the flour into the wet ingredients until a shaggy dough forms.
Turn onto a floured surface and knead for 8-10 minutes until smooth and elastic. The dough should be soft but not sticky.
Wrap in plastic wrap and let rest for 30 minutes at room temperature.
Prepare the Filling:
Boil the cubed potatoes in salted water until fork-tender, about 15-20 minutes.
Drain thoroughly and immediately mash while hot until completely smooth. No lumps!
Stir in the farmer's cheese, butter, salt, and white pepper until well combined. Taste and adjust seasoning. Let cool completely.
Assemble the Pierogi:
Divide the dough into 4 portions. On a floured surface, roll each portion to 1/8-inch thickness.
Using a 3-inch round cutter or glass, cut circles from the dough. Re-roll scraps as needed.
Place 1 tablespoon of filling in the center of each circle. Brush edges with water.
Fold the dough over to create a half-moon shape and crimp edges firmly with a fork or your fingers to seal completely.
Cook and Serve:
Bring a large pot of salted water to a gentle boil. Working in batches, add pierogi and cook until they float to the surface, about 3-4 minutes.
Meanwhile, caramelize the onions: In a large skillet, melt butter over medium heat. Add sliced onions and cook, stirring occasionally, until golden brown and jammy, about 15-20 minutes.
Remove pierogi with a slotted spoon and add directly to the skillet with onions. Gently toss to coat.
Serve immediately topped with caramelized onions, fresh chives, and dollops of sour cream.
Notes
Storage tip: Uncooked pierogi can be frozen for up to 3 months. Arrange on a floured baking sheet, freeze until solid, then transfer to freezer bags. Cook directly from frozen, adding 2-3 extra minutes to boiling time. Leftover cooked pierogi can be pan-fried in butter until crispy for a delicious next-day treatment.