400g seasonal mushrooms, roughly chopped
1tsp picked thyme
sea salt and freshly ground black pepper
450g piece of prime beef fillet or chateau
1-2tbsp Dijon mustard
500g ready-made puff pastry
2 egg yolks, beaten
Put the mushrooms into a blender with some seasoning and pulse to a 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. Heat in a frying pan and add a little olive oil. Season the beef and sear in the hot pan for 30 seconds on each side until nicely browned . 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. With a palette knife, spread the mushroom paste out then place the seared beef fillet in the middle. Keeping a tight hold of the cling film from the edge, neatly roll the 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.
Roll out the puff pastry on a floured surface to a large rectangle. Remove the cling film from the beef, then lay in the centre. Brush the surrounding pastry with egg yolk. Fold the ends over, the wrap the pastry around the beef, cutting off any excess. Press down the edges with a fork. Place on a baking sheet. Brush over all the pastry with egg and chill for about 15 mins to let the pastry rest. Heat the oven to 190deg C and cook for 22mins. Remove from the oven and let rest for a further 5 mins.