Make the dough: in a large bowl combine All-Purpose Flour with a pinch of Salt. Make a well, add a beaten Egg, 2–3 tbsp Neutral Oil and about 1/2 cup warm Water (add more if needed) and mix until a shaggy dough forms.
Turn the dough out onto a lightly floured surface and knead until smooth and elastic, about 6–8 minutes. Cover and let rest for 20–30 minutes to relax the gluten.
Prepare the filling: peel and cut Starchy Potatoes into chunks and boil in salted water until very tender, then drain.
While the potatoes cook, finely chop Onion and sauté it in a knob of Unsalted Butter until translucent and golden; set aside.
Mash the drained potatoes with the sautéed onion, a few tablespoons of Unsalted Butter (or the butter left in the pan), crumbled Farmer's Cheese, and season to taste with Salt and freshly ground Black Pepper. The filling should be creamy and spreadable but not too wet.
Roll the rested dough on a floured surface to about 1/8-inch thickness. Use a 2.5–3 inch round cutter to cut circles.
Place about 1 teaspoon (or a heaping teaspoon for larger circles) of filling in the center of each circle. Fold the dough over to form a half-moon and press the edges together, sealing firmly; crimp or pinch to ensure they don’t open while cooking.
Bring a large pot of salted water to a gentle boil. Cook the vareniki in batches so they have room to move; when they rise to the surface, cook 2–3 minutes more until heated through.
Remove with a slotted spoon to a platter. Toss immediately with melted Unsalted Butter or quickly sauté in a pan for a light golden finish if desired.
Serve the vareniki hot with plenty of melted butter and a generous spoonful of Sour Cream on the side for dipping.
