Pat the beef dry with paper towels and season well with salt and pepper. Heat half of the oil and 1 tablespoon of the butter in a large frying pan over high heat. Add the beef and cook for 5 minutes on all sides, or until well seared. Remove to a plate and allow to cool.
Place the mushrooms in a food processor and process until very finely chopped. Heat the remaining oil and remaining butter in a frying pan over high heat. Add the shallots and sweat for 1 minute, then add the mushrooms and cook for 15 minutes, or until all the liquid has evaporated and the mushroom mixture is very dry. Remove from the heat, season to taste, then add the thyme leaves. Set aside to cool for 15 minutes.
Arrange half the prosciutto slices on a sheet of plastic wrap so they are as wide as the tenderloin – the slices should overlap slightly. Spread mushrooms over the prosciutto. Lay the beef on mushrooms and roll the plastic wrap over to cover the beef. Refrigerate for 1 hour.
Preheat the oven to 220ºC.
Remove the plastic wrap and lay the prosciutto-wrapped beef in the centre of the pastry sheet. Brush the egg wash along the sides of the pastry, fold the pastry over to cover the beef, trim the excess pastry, then fold and seal each end to make a neat parcel. Brush the top with the egg wash. Add some cut pastry leaves for decoration and brush with the egg wash. Use the tines of a fork to press and seal the edges of the pastry and refrigerate for 10 minutes.
Place the beef in the oven and cook for 5 to 10 minutes until golden, then reduce the oven to 140ºC, opening the door for a minute to help lower the temperature. Cook for a further 20 minutes until you have a nice, juicy pink result. Rest for 5 minutes.
Place the beef on a cutting board, cut both the ends, then cut in tow in the middle and each half in two again to achieve four portions. Place each portion on a warm plate and serve with red wine sauce and sautéed green beans.
Red Wine Sauce
Heat the oil in a large saucepan over high heat.
Add the shallots and onion and cook for 5 minutes or until the onion caramelizes.
Add the peppercorns, mushroom and garlic and cook for 2 minutes.
Add half the wine, and simmer until nearly dry.
Pour in the remaining wine and cook for 10 minutes, or until reduced by three-quarters.
Add the stock, bring to the boil, then reduce the heat and simmer for 20 minutes, or until reduced by half, skimming the surface regularly.
Strain through a fine sieve into a small saucepan and bring back to a simmer, skimming again to make sure the sauce is free from impurities.
Simmer for 10 minutes, or until it is glossy and has the consistency of pouring cream.
Remove from the heat, add the herbs, cover with plastic wrap and set aside for 10 minutes to allow the flavours to infuse.
Pass through a fine sieve to remove the herbs and keep the sauce warm until needed.
Any excess sauce can be stored in an airtight container in the refrigerator for up to 5 days.