Pulse the mushrooms in a food processor with some seasoning to a rough paste. Scrape the paste into a pan and cook over a high heat for 8-10 minutes, tossing frequently, to cook out the moisture from the mushrooms. Spread out on a plate to cool.
Heat a frying pan and add the olive oil. Season the beef and sear in a hot pan for 30 seconds only on each side – just enough to colour it. Remove the beef from the pan and leave to cool, then brush all over with the mustard.
Lay a sheet of cling film on a work surface and arrange the Parma ham slices on it, in slightly overlapping rows. With a palette knife, spread the mushroom paste over the ham, then place the seared beef fillet in the middle. Keeping hold of the cling film from the edge, neatly roll the Parma ham and mushrooms around the beef to form a tight barrel shape. Twist the ends of the cling film to secure. Chill for 15-20 minutes to allow the beef to set and keep its shape.
Roll out the puff pastry on a lightly floured surface to a large rectangle, the thickness of a £1 coin. Remove the cling film from the beef, then lay it in the centre of the pastry. Brush the surrounding pastry with egg yolk. Fold the ends over, then wrap the pastry around the beef, cutting off any excess. Turn over, so the seam is underneath, and place on a baking sheet.
Brush the pastry all over with the beaten egg and chill for about 15 minutes to let the pastry rest. Heat the oven to 200˚C/Gas Mark 6.
Lightly score the pastry at 1cm intervals and glaze again with egg. Bake for 20 minutes, then lower the oven setting to 180˚C/Gas Mark 4 and cook for another 15 minutes. Allow to rest for 10-15 minutes before slicing and serving. The beef should still be pink in the centre when you serve it.
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This recipe is taken from Rays of Sunshine's first cookbook; A dish for a Wish. All the profits from the cookbook help the charity to continue granting magical wishes every day for seriously ill children. Click here to find out more about the book >