In a large bowl, whisk together flour and salt. Add the egg, sour cream, water, and melted butter. Stir until a soft dough forms.

Transfer dough to a lightly floured surface and knead until smooth, about 5 minutes. Cover with plastic wrap and let rest for 30 minutes.

While the dough rests, place potatoes in a pot of cold salted water. Bring to a boil and cook until tender, 15–20 minutes. Drain well.

Mash potatoes with farmer’s cheese, season with salt and pepper, and set aside.

In a skillet over medium heat, melt 2 tablespoons butter. Sauté chopped onion until golden and fragrant, then fold into the potato-cheese mixture.

Roll the dough on a floured surface to 1/8-inch thickness. Use a 3-inch round cutter to cut circles.

Place a tablespoon of filling in the center of each circle. Fold dough over and press edges firmly with a fork to seal.

Bring a large pot of salted water to a gentle boil. Cook perogies in batches; once they float, remove with a slotted spoon (about 2–3 minutes per batch).

Optional: For crispy edges, pan-fry boiled perogies in butter until lightly golden.

Serve hot, garnished with caramelized onions, a sprinkle of chives, or a dollop of sour cream.