Whisk the flour and salt in a large bowl. Add the egg, sour cream, and softened butter and mix until a shaggy dough forms, adding water 1 tablespoon at a time if it seems dry.Knead for 3-4 minutes until smooth and soft, then wrap and let it rest for 30 minutes.
Boil the potatoes in salted water for 15 minutes until tender, then drain well. Mash them with the butter, and while still hot, stir in the dairy free cheddar, onion powder, salt, and pepper until the cheese melts in. Let the filling cool completely.
While the dough rests, melt 3 tablespoons of butter in a large skillet over medium heat. Add the sliced onion and cook for 15 minutes, stirring occasionally, until deeply golden and sweet. Transfer to a bowl and keep the skillet handy.
Roll the dough out on a floured surface to about 1/8 inch thick and cut out 3 inch rounds with a cookie cutter or glass.Place about 1 tablespoon of filling in the center of each round, fold it in half, and pinch the edges firmly to seal.
Bring a large pot of salted water to a boil and cook the pierogi in batches. Once they float, cook for 2-3 minutes more, then lift them out with a slotted spoon.
Return the onions to the skillet over medium heat, add the boiled pierogi, and pan fry for 2-3 minutes per side until golden and crisp. Serve with dairy free sour cream and fresh chives.