1. Put the mushrooms into a food processor with some seasoning and pulse to rough paste. Scrape the paste into a pan and cook over a high heat for about 10 mins, tossing frequently, to cook out the moisture from the mushrooms. Spread out on a plate to cool.
2. Heat in a frying pan and add a little olive oil. Season the beef and sear in the hot pan for 30 secs only on each side. (You don't want to cook it at this stage, just colour it.) Remove the beef from the pan and leave to cool, then brush all over with the mustard.
3. 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 pace the seared beef fillet in the middle. Keeping a tight 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 mins to allow the beef to set and keep its shape.
4. Roll out the puff pastry on a floured surface to a large rectangle, the thickness of a £1 coin. Place the pastry sheet on a sheet of cling wrap. Remove the cling film from the beef, then lay in the center. Fold the ends over, then wrap the pastry around the beef, cutting off any excess. Roll up the dough in the cling wrap and tighten up the ends, just like before with the meaty part that went into the fridge. The tightened dough package should look like a small football. Place in the refrigerator and chill for about an hour (could be overnight).
5. Heat the oven to 200C/395F.
6. Glaze dough football with beaten egg yolk. With the back of the knife, lightly score the pastry at 1 cm intervals. Bake for 35 mins then allow to rest for 10-15 mins before slicing and serving with accompaniments. The beef should still be pink in the center when you serve (you can use an instant thermometer and remove the beef from the oven when it registers 120F).

Notes:

• Place beef on parchment paper—helps keep the bottom from getting soggy.