Sieve the flour and salt into a bowl and create a well in the center.
Whisk the egg, soured cream, and 30ml water together, then pour into the flour well.
Mix into a rough dough, then knead on a floured surface until smooth and non-sticky.
Wrap the dough in clingfilm and let it rest for 1 hour.
Soak dried mushrooms in hot water for 10 minutes.
Melt 20g butter and fry half the onion, field mushrooms, and garlic until dry.
Add the rinsed, chopped sauerkraut and chopped soaked mushrooms to the pan.
Add mushroom soaking liquid and 200ml water, then simmer for 20 minutes until dry.
Stir in parsley and dill, then remove from heat to cool.
Roll dough to 1-2mm thickness and cut into 7cm rounds.
Place 15ml of filling on each round and seal the edges into a crescent shape.
Boil pierogi in salted water for 5 minutes until they float, then drain.
Fry the remaining onion in 20g butter, add breadcrumbs, and cook until crisp.
Garnish the pierogi with onion breadcrumbs and chives to serve.