Ingredients
1 beef fillet weighing 1kg 
Olive oil 
50g butter 
250g mushrooms, I like a mixture of chestnut and portobello, either chopped with a knife very very finely or slowly pulse in a food processor until very finely chopped 
50g chestnuts
1 tsp porcini powder
1 large sprig fresh thyme 
100ml Prosecco  
12 slices prosciutto 
500g puff pastry (if you want to use ready rolled get 2 packets) 
1 egg, beaten 

Method
Heat the oven to 200c.   

When the oven is hot, put the beef in a roasting tray and put some olive oil over using your hands. Season with salt and pepper and roast for roast for 15 minutes for medium-rare or 20 minutes for medium. Remove from the oven and cool.   

When cool, wrap in cling film and chill in the fridge for 20 minutes.

Meanwhile make the duxelles.  Heat 2 tbsp of olive oil and 50g butter over a medium heat in a large frying pan or wok.  Add the mushrooms, chestnuts, porcini powder and thyme and cook for about 10 minutes until the mushrooms are soft and the liquid gone.  Pour in the Prosecco and continue to cook for another 10 minutes or so until all of the wine has been absorbed. The mixture should be soft and should have no liquid left.  Remove the thyme and put into a bowl to cool.

When the beef and mushrooms have cooled, remove the beef from the cling film and pat dry on kitchen towel. Cut off the string from your fillet carefully and discard your cling film. Put 2 long pieces of clingfilm down.  What you now need to do is lay the prosciutto on to the cling film, it needs to be the length and width of the beef fillet as all of it needs to just fit so work out how long and wide it needs to be and then lay the 12 slices prosciutto on the cling film, overlapping them in a double row so everything is fully covered.  With your hands spread the duxelles over the prosciutto, pushing it down so everything is fully covered and there are no gaps.  Sit the fillet on the top and, using the cling film for leaverage, carefully wrap the duxelles/prosciutto over the beef and then gently roll around the beef carefully.  All of the beef should now be covered with the proscuitto. Fold the cling film over so that it covers everything. Now bunch the sides of the clingfilm by grabbing the ends and using them as a handle. Carefuly roll the clingfilm so that both ends tightens.  Tie the ends. Put into the fridge for at least 10 minutes.

Cut the pastry one third/two thirds and roll out.  The small piece of pastry needs to be rolled out thinly and needs to be big enough for the fillet to sit and have an extra 5cm all around the edge.  Place on a non-stick baking sheet.  Mix the egg yolks with the water and brush the edge of the smaller piece of pastry.  Put the beef fillet on the top making sure it sits perfectly in the centre.  Brush the top and sides of the prosciutto with the egg. 

The larger piece of pastry needs to be rolled large enough to cover the beef and meet the edge of the smaller piece.  Put over the top of the fillet, using your hands to smooth it over the fillet so it sits tightly.  Cut the pastry so there is 5cm around and push the edges of the top and bottom of the pasty together.  I like to use a spoon or knife handle. 

Brush the egg wash over the top of your beautiful parcel and then chill in the fridge for at least half an hour (or up to 24 hours).  

When you are ready to cook, preheat the oven to 180c.  Cover with egg wash again and put into the oven until the pastry is golden and puffy.

Rest for 10 minutes before cutting. 

- Original recipe taken from here