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1
Heat oven to 220C/fan 200C/gas 7.
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2
Sit the 1kg beef fillet on a roasting tray, brush with 1 tbsp olive oil and season with pepper, then roast for 15 mins for medium-rare or 20 mins for medium.
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3
When the beef is cooked to your liking, remove from the oven to cool, then chill in the fridge for about 20 minutes.
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4
While the beef is cooling, chop 250g chestnut (and wild, if you like) mushrooms as finely as possible so they have the texture of coarse breadcrumbs.
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5
You can use a food processor to do this, but make sure you pulse-chop the mushrooms so they don't become a slurry.
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6
Heat 2 tbsp of the olive oil and 50g butter in a large pan and fry the mushrooms on a medium heat, with 1 large sprig fresh thyme, for about 10 mins stirring often, until you have a softened mixture.
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7
Season the mushroom mixture, pour over 100ml dry white wine and cook for about 10 mins until all the wine has been absorbed.
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8
The mixture should hold its shape when stirred.
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9
Remove the mushroom duxelle from the pan to cool and discard the thyme.
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10
Overlap two pieces of cling film over a large chopping board.
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11
Lay 12 slices prosciutto on the cling film, slightly overlapping, in a double row.
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12
Spread half the duxelles over the prosciutto, then sit the fillet on it and spread the remaining duxelles over.
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13
Use the cling film's edges to draw the prosciutto around the fillet, then roll it into a sausage shape, twisting the ends of cling film to tighten it as you go.
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14
Chill the fillet while you roll out the pastry.
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15
Dust your work surface with a little flour.
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16
Roll out a third of the 500g pack of puff pastry pastry to a 18 x 30cm strip and place on a non-stick baking sheet.
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17
Roll out the remainder of the 500g pack of puff pastry to about 28 x 36cm.
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18
Unravel the fillet from the cling film and sit it in the centre of the smaller strip of pastry.
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19
Beat the 2 egg yolks with 1 tsp water and brush the pastry's edges, and the top and sides of the wrapped fillet.
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20
Using a rolling pin, carefully lift and drape the larger piece of pastry over the fillet, pressing well into the sides.
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21
Trim the joins to about a 4cm rim.
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22
Seal the rim with the edge of a fork or spoon handle.
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23
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.
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24
Chill for at least 30 mins and up to 24 hours.
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25
Heat oven to 200C/fan 180C/gas 6.
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26
Brush the Wellington with a little more egg yolk and cook until golden and crisp - 20-25 mins for medium-rare beef, 30 mins for medium.
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27
Allow to stand for 10 mins before serving in thick slices.
