For the Dough:
Mix the eggs, egg yolk, sour cream and 125ml water together in a bowl.
In another bowl, combine the flour and 2 teaspoons salt.
Mix the wet ingredients with the dry ingredients, adding the remaining water slowly and in increments until the mixture is adequately hydrated. Mix until soft and smooth texture.
Cover dough. Refrigerate for 15 minutes.
For the Filling:
Preheat the oven to 80°C. Peel and cut potato into thirds and place into a large saucepan. Bring the water to a boil and cook until fork tender.
Place the potatoes on sheet trays and place in oven until all moisture has been released and no more steam rises.
Small dice and caramelise the onions in a large skillet. Add the potatoes, cheese and egg yolks and stir to combine. Season with salt and pepper.
Using a pasta sheeter attachment on a stand mixer or using a rolling pin, sheet the pierogi dough to setting number 3 or to your desired thickness.
Using a rim of a glass, cut out circles into the sheets of dough.
Place the maximum amount of filling into the cut-out circle that will allow you to fold over the dough, pinch the edges, and shape into half moons.
Bring a saucepan of water to a boil, and put the pierogi inside and poach on a rapid simmer for 2 to 3 minutes.
Alternatively you could pan-fry your pierogis in clarified butter.