Once wrapped place in the fridge to cool. The butter in the mushroom mix will help it firm up so that it will stay in place once the plastic wrap is removed.
On a lightly floured surface gently roll out your puff pastry.
Place the chilled beef onto the pastry.
Brush a little egg wash around the beef then fold the pastry over. Use a fork or the back of a knife to crimp and seal the edges. Then apply aug wash over the pastry.
Sprinkle with salt (we used a black lava salt here) then bake in a 390f preheated oven for about 20 minutes. Turn the pan half way through the cooking to ensure an even golden brown.
Remove from the oven and let rest for at least 5 minutes then Carefully slice into portions
The beef should be a lovely medium rare with a coating of moist mushrooms and a deliciously crispy flakey pastry crust. Serve with a few vegetables.
Pour over a little gravy or jus.