Celeriac, mushroom & Cheddar pie

Boil a kettle. Put the dried porcini in a small bowl and cover with 250ml just-boiled water, then soak for 30 minutes.

Meanwhile, thinly slice the leeks, then peel and cut the celeriac into 1.5cm cubes.

Heat the oil in a large saucepan over a medium heat. Add the leeks, celeriac and salt, then cook for 10 minutes, stirring often, until the leeks have softened.

When the porcini mushrooms have soaked, scoop out of the soaking water (keep the water) and chop finely. Add the garlic to the celeriac mixture, then the chestnut mushrooms and chopped porcini. Stir together and cook for 5 minutes, then add the porcini water. Cover and simmer for 15 minutes or until the celeriac is soft.

Add the black pepper, then mix in the mustard and cheese until melted. Take off the heat for a couple of minutes, then stir in the cream and tarragon.

Season if needed, then tip into a roughly 20x32cm pie or gratin dish. Leave to cool slightly.

Brush the rim of the dish with beaten egg and lay the pastry over the top of the filling, pressing down around the edge. Trim to fit, then crimp with a fork or your fingers. Decorate with pastry trimmings if you fancy and cut 2-3 slits in the top to allow the steam to escape. Brush with egg and bake for 30 minutes, until the pastry is puffed and golden and the centre is piping hot.