Combine the flour, butter, egg and a little water, mix with your hands gradually adding more water. Knead the dough until smooth. Divide the dough into 3 parts. On a lightly floured surface thinly roll out each part.
Cut the dough into 4cm squares, place a teaspoon of filling in the centre of each square. Fold over the filling to form a triangle. Pinch the edges to seal and bring together the opposite endings of triangle and pinch.
For the filling:
Rinse the mushrooms, soak for 15 minutes and cook in a large pan of salted water until tender. Remove from the pan with a slotted spoon and and chop. Reserve the liquid.
In a frying pan, warm the oil then add the onions, cook and stir until soft.
Stir in mushrooms, soured cream, breadcrumbs and mix well. Simmer for few minutes, season with salt and pepper to taste.
Reheat the water the mushrooms were cooked in. Place dumplings in batches and cook until they float on the surface. Spoon into a colander and drain. Serve.