Pat the fillet dry with kitchen paper and trim any thin ends (fold it back if you don’t want to waste any meat) to form a relatively even log shape. Season all over with salt & pepper, then heat the oil in a wide pan and sear on all sides for a minute or two, then remove from the heat and transfer to a plate. The meat shouldn’t be cooked yet, you just want some colour on the outside. While the meat is resting, prepare a sheet of plastic wrap on a working surface. Line the plastic with a layer of slightly overlapping prosciutto, the same width as the length of the fillet. Spread the prosciutto layer thinly with paté (warm it slightly if the paté is too stiff ), then place the fillet on top and roll up snugly, keeping the plastic on the outside. Twist the ends and refrigerate for 30 minutes to firm up. In the meantime, make the mushroom sauce: In a wide pan, melt the butter over medium high heat, then fry the mushrooms until lightly browned and soft, seasoning with salt & pepper. Add the cream and bring to a simmer, then cook for about 5 minutes over low heat. Set aside to reheat before serving.

Preheat the oven to 220 C. Line a baking tray with non-stick baking paper. On a clean working surface, roll out the puff pastry. Remove the firmed fillet from the fridge and remove the plastic. Place it on top of the pastry and roll up neatly, tucking the ends and keeping the seam on the bottom. Use a sharp knife to score the pastry lightly on top taking care not to cut all the way through, then place it on the lined baking tray and brush all over with beaten egg. Bake for 25-30 minutes for medium rare, depending on the thickness of your fillet. If you’re using a thinner/smaller fillet, reduce the baking time. When cooked, the pastry should be golden all over. Let the Wellington rest for at least 10-15 minutes before slicing, then serve with warm mushroom sauce.