Make the dough: In a large bowl combine the All-Purpose Flour and a pinch of Salt. Make a well, add warm Water, the Egg and Oil, and mix until a shaggy dough forms; turn out and knead 6–8 minutes until smooth and elastic. Wrap and let rest 30 minutes.
Prepare the filling: Peel and quarter the Starchy Potatoes and boil in salted water until very tender. Drain and mash until smooth, then fold in crumbled Farmer's Cheese. In a small skillet, melt Butter and sauté finely diced Onion until golden; stir most of the onion into the potato mixture and season with Black Pepper and more salt to taste.
Assemble pierogi: Roll the rested dough on a lightly floured surface to about 1/8" (3 mm) thickness. Cut rounds (about 3"/7–8 cm) and place a teaspoon or so of filling in the center of each round. Fold dough over to form a half-moon and press the edges to seal, crimping with a fork if desired.
Cook pierogi: Bring a large pot of salted water to a gentle boil. Drop pierogi in batches so they don’t crowd; when they float, cook 1–2 minutes more, then remove with a slotted spoon to drain.
Optional finish: For extra flavor and texture, heat an additional knob of Butter in a skillet and sauté remaining sliced Onion until caramelized. Add drained pierogi and fry a few minutes per side until lightly browned and crisp.
Serve: Plate the pierogi warm with a generous spoonful of Sour Cream and a sprinkle of chopped Chives. Adjust seasoning with extra Salt and Black Pepper if needed.
