Dice the bacon and onion and fry both in a frying pan until golden brown. Wipe and thinly slice the mushrooms, then add them to the onion and bacon with the tomato puree, celery salt, pepper and butter.

Fry until all the liquid has evaporated, stirring frequently.

Roll out the pastry on a floured board to about the thickness of the blade of a knife, then cut it into 12 rounds. Spoon the mushroom mixture into the centre of each round. Brush the edges with water, fold the pastry over and press the edges firmly together, using a fork.

Rinse a baking tray under cold water. Beat the egg yolks. Arrange the piroshki on the wet baking tray and brush them with beaten egg. Prick the pastry several times with a fork to allow steam to escape.

Bake in a moderate oven (180 c, 350f, gas 4) for 25 minutes and serve hot.