Preheat the oven to 200°C.

In a bowl, lightly mix the potato pieces with a good splash off Olive Oil, a teaspoon of Thyme, and some Sea Salt and Black Pepper. Place the seasoned pieces onto a baking tray, cover with foil and bake for 45 minutes or until soft. Leave to cool completely.

Bring a large pan of salted water to a boil. Add the spinach, and blanch for 1 minute. Drain and refresh under cold water, then squeeze out as much excess water as possible. Roughly chop and set aside.

Prepare ahead: chop and soak the nuts overnight to soften.

Heat the butter and Olive Oil in a large pan and gently cook the leeks and shallots for 5 minutes until softened and caramelized. Add the rest of the Thyme and garlic, fry for a further minute then add the mushrooms and fry until softened and all of the liquid has evaporated. Remove from the heat, stir in the nuts and Ground Nutmeg and leave to cool. (A dry, cool mixture is important to stop the pastry from becoming soggy.)

Transfer the cooled mixture into a large bowl and stir in the cheese, spinach leaves and potato pieces. Lay a large sheet of clingfilm on the work surface and tip the mixture into the middle. Mould it into a log shape, then roll it up tightly in the clingfilm to make a 26cm-long cylinder. Place the mixture in the fridge for at least 1 hour, until firm.

Dust your work surface with flour and roll the pastry into a 34 x 28cm rectangle. Lay the cylinder of filling on top and unwrap. Whisk together the egg yolk and milk, and brush the mixture along the edges of the pastry.

Carefully roll the filling in the pastry so that the join is on the underside. Press the ends together to seal and crimp with a fingertip or a fork. Cut any pastry trimmings into festive shapes and use them to decorate the surface.

Place the wellington on a baking sheet and brush with the remaining egg and milk mixture. Bake for 30 minutes until the pastry is golden brown and crisp.

Leave to stand for 5 minutes before slicing.