Boil the potatoes, skins on, until a knife slides in easily and the potato falls off the knife back into the water. Let them cool a bit While the potatoes are cooling, saute the onions in olive oil until translucent, soft and shiny, not browned.
Peel potatoes, and cut into chunks into a large bowl.
Mash. Add the sauteed onions, mixing thoroughly.
Add sour cream, mix well.
Season to taste with salt, pepper, horseradish and dill. - I like enough horseradish to get my sinuses open, but not so much that my eyes water.
Combine the yeast, sugar and lukewarm milk in a large bowl and let sit until the yeast gets foamy, about 5 minutes. It should smell nice and yeasty and look brown and puffy.
Add the melted butter, salt and beaten egg to the yeast mixture, stirring very well to incorporate all that butter.
Add 3 ¼ cups of flour, 1 cup at a time, stirring very well after each addition. You should have a pretty rough dough by now.
Turn out the doughy into a well-floured board and knead for 5 minutes or so, adding more flour as necessary to keep it from sticking.
Cover with a tea towel and let rest for 10 minutes. Use it now or refrigerate it for up to 24 hours.
Preheat the oven to 350 F.
Line a baking sheet with parchment or a silpat baking mat.
With a pastry scraper, divide the dough into thirds. With a rolling pin, roll out one third of the dough about 1/8 inch thick.
Using a round biscuit or cookie cutter (or the top of a drinking glass), cut out rounds of dough.
Place a fairly heaping spoonful of filling in the center of the circle, then fold the dough over, so it makes a plump semi-circle.
Press the edges together firmly and crimp closed with the tines of a fork.
Place on the baking sheet.
Mix together the egg yolk and milk, and brush the pirozhki with the egg wash.
Bake for 25-30 minutes until golden brown and delicious.