• Preheat the oven to 400℉/200℃ (no fan)

Make the mushroom filling

• Soak the porcini mushrooms in boiling water for 10-15 mins (and keep the water). Sautée the diced 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.

Assemble and bake

• Trim any silver skin from the pork tenderloin and make a slit all the way down the middle lengthwise with a knife. Season all over with salt and pepper. Stuff the slit with the cooled mushroom filling and reform it together. Wrap it in plastic food wrap to help keep its shape and place it in the fridge if you are not using it straight away.
• Roll the pastry out (if necessary) 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 so that they cover the whole surface, but leave a small margin at the edges.
• Place the stuffed pork tenderloin lengthwise onto the prosciutto and puff pastry sheet. Lift up each long side around the loin, overlapping the edges slightly and pressing them together before brushing seams with egg wash to seal them. Now fold the two short sides up and press them together to form a parcel, tucking in the sides and sealing with more egg wash, trimming any excess.
• Gently turn the tenderloin wellington over so that the long seam is on the bottom. Now brush all over with egg wash.
• Bake the Wellington at 400F°/200°C for 20 minutes until golden brown, then remove to a cooling rack for 10-15 minutes to rest.

Make the pan sauce

• Heat the same pan used to make the mushrooms 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 a third to thicken it.
• Finally, stir in the mustard and more chopped sage and thyme. Season with salt and pepper. You can add a squeeze of lemon juice if it needs brightening.
• Slice up the pork wellington and drizzle in the mushroom cream sauce.