Soak the porcini mushrooms in boiling water for 10-15 mins (and keep the water). Sautée the fresh mushrooms and 1 shallot in butter until cooked before adding the porcini, and then keep the heat on medium/low until all the moisture evaporates. Now transfer to a bowl and cool completely (in the fridge/freezer if necessary). Set the pan aside but don’t clean it as we will reuse it for the sauce.
Trim any silver skin from the pork tenderloin and make a slit all the way down the middle lengthwise with a knife. Stuff the slit with the cooled mushroom filling and reform it together. Wrapped in clingfilm to help keep its shape.
Roll the pastry out into a rectangular shape big enough to fit around the pork tenderloin. Score a crisscross pattern on the pastry lightly with a knife and then flip over and place onto a baking tray.
Lay the prosciutto slices directly on the pastry.
Place the stuffed pork loin lengthways onto the prosciutto and puff pastry. Lift up each long side around the loin, pressing together the edges and brushing seams with egg wash. Then roll the 2 short sides up and press them together to form a parcel, tucking in the sides and sealing with more egg wash, trimming any excess. Brush all over with an egg wash.
Bake at 425F /220C for 20 minutes until golden, then remove to a cooling rack for 15 minutes.
Meanwhile, make the sauce. Heat the same pan used to make the stuffing and melt some butter into it. Add diced shallot and sauté for 1 minute until soft. Deglaze with the white wine, then add the reserved porcini water and cream before leaving it to reduce by ⅓ and stirring in the mustard.
Slice up the pork wellington and drizzle in the mushroom cream sauce.