Whisk the flour and salt in a large bowl. Make a well, add the egg, sour cream and melted butter, and stir with a fork until shaggy.

Knead for 5 to 7 minutes until smooth and soft, like an earlobe. If it feels dry and cracks, add warm water a tablespoon at a time. If it sticks, dust with a little flour.

Wrap the dough and let it rest at room temperature for 30 minutes (or refrigerate up to 2 days and bring back to room temperature before rolling).

Boil the potatoes in salted water until very tender, 12 to 15 minutes. Drain well and let them steam dry for a couple of minutes.

Cook the diced onion in 1 tablespoon butter until soft and golden. Mash the potatoes with the onion and cheddar, season well, and let the filling cool completely.

Roll half the dough on a lightly floured surface to about 1/16 to 1/8 inch thick. Cut 3-inch rounds. Re-roll scraps once.

Place a heaping teaspoon of filling in the center of each round, fold in half and pinch the edges firmly to seal. Keep finished pierogi under a towel.

Boil in a large pot of salted water, about 10 at a time. Once they float, cook 2 to 3 minutes more, then lift out with a slotted spoon.

Melt 3 tablespoons butter in a skillet, cook the sliced onion until golden, add the boiled pierogi and fry 2 to 3 minutes per side until crisp. Serve with sour cream and chives.