Sit the beef in a roasting tray and brush with a little olive and season with salt and pepper.
Place in an oven at 220ºc for around 20 minutes for medium or 30 minutes for a more well done meat. Remove from the oven and allow to cool before chilling in the fridge.
Chop the mushrooms finely – a food processor can be used for this. They need to be like breadcrumbs.
Heat the butter with two tablespoons of olive oil in a pan and cook the mushrooms with the sprig of thyme for around 10 minutes stirring regularly. Season with salt and pepper then add the wine to the pan. Cook for a further ten minutes until the wine has been absorbed. It should hold its shape when done and not be runny. Remove from the heat and discard the thyme. Allow to cool. Place two large pieces of clingfilm over a large chopping board and lay the prosciutto across the film. Spread half of the mushroom mixture onto the prosciutto then place the cooled beef on top. Top with the remaining mushroom mixture. Use the clingfilm to roll the prosciutto around the beef and wrap tightly. Place back in the fridge to chill. Line a baking tray with parchment and dust with flour. Place one of the unrolled sheets of pastry onto the baking sheet. Remove the beef from the clingfilm and sit onto the pastry. Brush the egg wash all around the edges of the pastry and the top and side of the beef. Carefully drape the second sheet of pastry over the beef.
Cut the excess pastry from the base sheet and set aside. Seal the edges with a fork. Using the spare cuts of pastry cut out leaf shapes and stick to the top of the wellington with egg wash. Chill for a minimum of 30 minutes and upto 24 hours.
Heat the oven to 20 and brush all over with egg wash. Place in the oven for 30 minutes for medium and 45 minutes for well done. If the pastry becomes too golden cover with foil to prevent burning. Leave to stand for ten minutes before serving in thick slices.