Preheat the oven to 200C/400F/gas mark 6. Grease a deep, 18cm loose bottomed cake tin. Spread the nuts out on a roasting tray and place under a grill, stirring occasionally, until golden. Set aside to cool.

Melt the butter in a large pan and add the onion and garlic, cook gently for about 10 minutes until translucent. Add the mushrooms and herbs and cook until the mushrooms are soft and any liquid has evaporated; set aside to cool.

Peel the parsnips, cut into chunks and cook in a pan of salted water until soft; drain then mash, set aside to cool.

Place the nuts in a food processor and blitz until they are finely chopped. Tip into a large bowl with the breadcrumbs, mix in the cheese, cooled vegetables and mashed parsnip and season generously with salt and pepper, taste, then mix in the beaten egg until well combined.

For the pastry, mix together the flour, salt and eggs in a large bowl. Place the butter and water in a pan and heat until the butter has melted and the mixture just comes to a boil. Quickly beat it into the flour mixture, a little at a time until it has all been combined and the mixture is smooth, it should be quite soft. Shape into a ball, wrap in clingfilm and pop in the fridge for 10 minutes until it has cooled to warm room temperature.

Give the pastry a very brief knead then set aside two fifths for the lid and decorations. Roll the remainder out on a lightly floured surface until it is 5mm thick. Carefully lift it into the tin, easing it into the corners; trim off the excess pastry, leaving 1cm hanging over the edge of the tin.

Spoon the filling into the tin and press it down. Brush the exposed pastry rim with the beaten egg. Roll out the remaining pastry to 5mm thick then cut out a circle 1cm wider than the tin. Place the pastry over the top of the pie and press the edges together to seal then crimp them. Cut a slit in the middle to let any steam escape.

Re-roll any leftover pastry and trimmings then cut out decorative shapes such as stars or holly leaves with a pastry cutter. Brush the underside of each with a little beaten egg then arrange them on top of the pie. Brush the whole of the top of the pie with beaten egg.

Bake for 1 to 1 ½ hours until hot all the way through, a metal skewer inserted into the centre should feel hot to the touch. Cover the top of the pie with tin foil after 20 minutes or so to prevent it from burning.

Leave the pie to cool in the tin for 10 minutes then carefully unmould it. If the sides are too pale, you can place it on a baking sheet and pop it back into the oven for 10-15 minutes (keeping the top covered) until it is golden. You can also brush the sides with beaten egg before you put it back in the oven if you want the whole pie to be shiny.