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First season the fillet all over and seal in a hot pan all over then roast for 8 mins for medium-rare or 10 mins for medium. When the beef is cooked to your liking, remove from the oven to cool, then chill in the fridge for about 15 mins.
While the beef is cooling, chop the mushrooms as finely as possible so they have the texture of coarse breadcrumbs.
Heat 2 tbsp of the oil and all the butter in a large pan and fry the mushrooms on a medium heat, with the thyme sprig, for about 10 mins stirring often, until you have a softened mixture. Season the mushroom mixture, pour over the wine and cook for about 10 mins until all the wine has been absorbed. The mixture should hold its shape when stirred. Remove the mushroom duxelle from the pan to cool and discard the thyme.
Overlap two pieces of cling film over a large chopping board. Lay the cured meat on the cling film, slightly overlapping, in rows.
Next add the blanched spinach. It is very important to dry this out beforehand, so spread the leaves on a tea-towel and place another one on top to soak up any excess water.
Spread the mushroom mix over the cured meat.
Coat the fillet with Dijon and place at the bottom half of the layers.
Use the cling film's edges to draw the cured meat around fillet, then roll it into a sausage shape, twisting the ends of cling film to tighten it as you go.
Roll out a third of the pastry to a 15 x 25cm strip and place on a non-stick baking sheet. Roll out the remaining pastry to about 25 x 30cm. Unravel the fillet from the cling film and sit it in the centre of the smaller strip of pastry and brush the pastry's edges, and the top and sides of the wrapped fillet, with beaten egg yolk. Using a rolling pin, carefully lift and drape the larger piece of pastry over the fillet, pressing well into the sides. Trim the joins to about a 4cm rim. Seal the rim with the edge of a fork or spoon handle. Glaze all over with more egg yolk and, using the back of a knife, mark the beef Wellington with long diagonal lines taking care not to cut into the pastry. Chill for at least 30 mins and up to 24 hrs.
Heat oven to 180℃ (with fan) 200℃ (without fan) or gas mark 6. Brush the Wellington with a little more egg and cook until golden and crisp - 20 mins for medium-rare beef, 25 mins for medium. Allow to stand for 10 mins before serving in thick slices. Serve with Beef Bones Gravy.