Polish
Boil the potatoes until tender, drain and mash smooth.
Finely chop and fry one onion in butter until golden, then mix into the potatoes with the cheese, salt and pepper to make the ruskie filling.
Knead flour, egg, warm water, sour cream and salt into a soft, smooth dough and rest covered 30 minutes.
Roll the dough thin, cut out 8 cm rounds, add a spoon of filling to each, fold and pinch the edges to seal tightly.
Boil the pierogi in batches in salted water for 3 to 4 minutes, until they float, then lift out.
Slice the second onion and fry in butter until golden.
Serve the pierogi topped with the fried onions and extra sour cream.