While still warm, add the flour and knead a dough. Roll out sheets to about 1/8" thick. Bake them in a preheated 392°F (200°C) oven about 8-10 min. It's preferable not to have more than 5 sheets.
After you've baked the sheets, beat the sour cream with the sugar (by hand, mixer would make it too runny). While sheets are still warm, arrange in the following order: sheet, cream, sprinkle walnuts, and so on until out of sheets. Optionally, add blueberry jam.
Cover cake with foil and put in the fridge for a few hours, best if overnight.