Preheat the oven to 180°C (350°F/Gas 4). Lightly grease a 20 cm (8 inch) spring-form cake tin.
Cut the kidneys into 1.5 cm (5⁄8 inch) pieces then devein.
Melt half of the butter in a frying pan over a medium heat.
Brown the kidneys, stirring occasionally, for around 5 minutes.
Transfer the kidneys to a colander to drain and leave to rest for 5 minutes.
Place the mushrooms in a heatproof bowl, cover with boiling water and leave to rehydrate for 5 minutes.
Drain then squeeze them in a clean tea towel (dish towel) to remove any excess moisture.
Melt the remaining butter in a clean frying pan over a medium heat.
Sauté the mushrooms, bacon, spring onion, carrot and parsnip.
Add the tomato paste and veal stock, then add the kidneys and bring to the boil.
Cook, stirring occasionally, for 5 minutes, or until the mixture reaches a syrupy consistency.
Season, then transfer to a bowl and cool slightly. Refrigerate until completely cooled.
Line the base and side of the tin with two pastry sheets, slightly overlapping.
Press the pastry sheets together to seal. Trim the pastry, leaving a 1 cm (1⁄2 inch) border above the tin. Refrigerate for 30 minutes.
Spoon the cooled filling into the lined tin. Cut a 20 cm (8 inch) circle from the remaining pastry.
Moisten the edge then place on top, pressing the pastry edges together to seal.
Trim any excess pastry and make a hole in the middle (it will act as a ‘chimney’).
Glaze the top of the pie with the egg yolk and bake for 45 minutes, or until golden.
Stand in the tin for 5 minutes. Serve hot.