Preheat the oven to 200ºC, gas mark 6. Grease a deep, round, 18cm loose-bottomed cake tin with a little oil.

Peel the celeriac, then cut into 1cm pieces and toss with a little lemon juice to prevent it from browning.

Cut the parsnip into 1cm pieces, then toss with the celeriac, 1 tbsp oil and some seasoning to coat.

Spread the vegetables out on a parchment-lined baking tray, then roast for 20 minutes, or until golden and tender.

Meanwhile, melt the butter in a large frying pan over a low heat, then add the leeks, thyme and some seasoning. Sauté for 10-12 minutes until soft.

Tip the leeks into a large bowl, add the roasted vegetables and mix well with the remaining filling ingredients, except the eggs and cranberry sauce.

Leave to cool for 20 minutes, then add the beaten eggs.

Meanwhile, make the pastry. Beat 2 eggs. Mix together the flour, salt and beaten eggs with a metal spoon in a large mixing bowl.

Add the butter and 180ml water to a saucepan and heat until it melts and the mixture just comes to the boil.

Quickly beat it into the flour mixture with a spatula, a little at a time, until combined smooth. It will be quite soft.

Shape into a ball, wrap and chill for 10 minutes.

Knead the pastry briefly, then cut off and set aside 350g for the lid and decorations.

Roll the remaining pastry into a ball, then roll out on a lightly floured surface until 0.5cm thick and at least 35cm in diameter.

Carefully lift into the tin and ease into the corners. Using a knuckle, gently press the pastry into the edges.

Trim off the excess pastry, leaving 1cm hanging over the edge of the tin.

Spoon the filling into the pastry-lined tin and gently press down.

Spoon the cranberry sauce evenly over the surface of the filling.

Beat the remaining egg, then brush some over the exposed pastry rim.

Roll out the remaining pastry to 0.3cm thick, then cut out a circle 1cm wider than the tin.

Place the pastry over the top of the pie, press the edges together to seal, then crimp between your thumb and forefingers.

Cut a small cross in the middle to let steam escape.

Reroll any leftover pastry and trimmings, then cut out decorative shapes with a pastry cutter.

Brush each shape with a little beaten egg, then arrange on top of the pie.

Brush the whole top of the pie with more beaten egg.

Bake for 20 minutes, then cover the top with foil and cook for 1 hour more, or until piping hot throughout and a metal skewer inserted into the centre feels hot to the touch.

Leave the pie to cool in the tin for 10 minutes, then carefully unmould it.

Brush the sides with beaten egg and place on a baking tray.

Return to the oven for 5-10 minutes (keep the top covered with foil if it looks dark) until golden and shiny all over.