In a large bowl: combine eggs, sourdough starter, water, baking powder and salt. Then add flour and mix the ingredients with a wooden spoon until a dough forms. Knead the Dough on a floured surface until it becomes a smooth and elastic ball. Cover and let it rest for 30-60 minutes.While the dough rests, you can simultaneously cook the potatoes.
Cook the Potatoes:
Peel, cut, and boil the potatoes in a pot of salted water. Drain once cooked.
Saute Onions
On medium heat, melt 1 stick of butter in frying pan. Chop up onions and add to the pan. Saute until golden brown.
Combine Potatoes and Onions
Mash the potatoes with this potato masher, fork, or hand masher until they are smooth. Mix ½ of the sauteed onions with potatoes, salt/pepper to taste, and a tablespoon of the melted butter from sauteed onions.
Assembly
First, prepare a large baking sheet or a cutting board dusted with flour. Assembling Varenyky: After the Dough has rested, split it into 4 equal parts. Roll out one part on a floured surface to ⅛" thickness, keeping the rest covered. Use a 2 inch cookie cutter or edge of glass to cut out circles of Dough. Place a small spoonful of the potato filling in the center of each circle of dough then fold the dough over the filling to create a half-moon shape. Pinch it firmly with fingers. Place varenyky on flour dusted surface (baking sheet or cutting board) until you are ready to boil a batch.
Cook the Varenyky
Bring a large pot of salted water to a boil. Carefully drop the dumplings into the boiling water in small batches, not to overcrowd, using a slotted spoon. Cook for about 8-10 minutes, or until they float to the surface. FYI: Because these are made with sourdough, it will take longer to boil and can take up to 10 minutes. Once they are cooked, remove the varenyky from water with a slotted spoon and place them into ½ of the remaining fried onion. Toss well and serve with sour cream.
Notes
If you'd like to fry the pierogi, you can do so after you've boiled them first.