Make pierogi dough. Whisk eggs, kefir, oil, start gradually adding flour, knead the dough by hand, keep adding flour till no longer sticky. Wrap in plastic wrap and let it rest for at least 10 minutes.
Make potato filling. Wash, peel, slice potatoes into large cubes. Place in the pot, fill with water, and boil till easily pierced. Don't overcook. Once cooked, drain, add the rest of the ingredients for the filling, mash so the texture is thick.
Make pierogi. Roll out the dough, using a cookie-cutter make several 1/8-in.-thick round shape circles. Place a teaspoon of potato filling on the center of each circle. Bring the opposite ends and pinch the dough in the middle, continue pinching from the middle to end, both sides, slightly press in your hands to make a flatter shape.
Cook pierogi. Fill a large pot with cold water, bring to boil. Once boiling, carefully adds pierogi, reduce to medium, once floating boil for 2 minutes. Remove them from the pot with a slotted spoon or strainer. Cook in parts, not all at the same time.
Fry (optional step). Heat cast-iron skillet on medium-high heat, once heated add ghee, and fry diced onions till golden, remove onions and set aside for now. Add pierogies, let them get crispy on one side before turning them over. Fry the pierogies in hot oil until they are golden brown. Once all pierogies are fried, top with fried onions.
Serve. Serve immediately with melted butter, sour cream, or yogurt.