Step 1 Preheat oven to 220’C/200’C fan/gas mark 7.
Step 2 Soak the dried porcini by covering in boiling water and leaving for 15-20 mins.
Step 3 Season the chateaubriand with salt and pepper. Heat the oil in a pan over a high heat to get it as hot as you can and sear the meat on all sides for approximately 2 mins on each side. Remove the beef, brush with the English mustard and allow to cool.
Step 4 Using the same pan melt the butter over a medium heat and add the chopped mushrooms and porcini mushrooms in their juice and thyme. Allow the mushrooms to brown for 5 mins then turn the heat up and add the Madeira or Port. Allow to bubble until the liquid has all but disappeared then remove from the heat and season to taste. Allow to cool and discard the thyme.
Step 5 Roll out the pastry quite thinly. Place the prosciutto slices so they overlap slightly in the middle of the pastry. Spread with the cooled mushroom mixture, making sure you leave a generous edge all the way round. Brush the edge with some of the beaten egg yolk. Place the beef in the middle and carefully roll the pastry around, sealing the edges and tucking the joined edges underneath, before carefully placing on a greased baking tray. Brush the pastry all over with the rest of the beaten egg yolk. Decorate the pastry by criss crossing or making shapes with any leftover pastry. Chill for at least 30 mins or up to 24 hrs.
Step 6 Pop in the pre heated oven for 30-35 mins until the pastry is golden. If the pastry is browning too quickly lower the heat a little. Allow to stand for 15 mins before serving in slices. Delicious served with Potatoes Dauphinoise and green beans.