Sift the flour and salt in a bowl. Cut up the cold butter and add it to the flour. Using your fingers, incorporate the butter into the flour until you have a coarse mixture. Add a bit of water at a time until the dough comes together in a ball. It will be coarse and sticky. But you don't want it overly wet. Cover and chill for 20 minutes.

On a floured surface, knead the dough until it is a smooth dough where you can see a marbling of the butter. Don't overwork the dough.

Roll out thinly into a rough rectangle. Fold into thirds in an envelope. Rotate 90 degrees and roll out again into rectangle. Fold again into thirds. Cover and chill for at least 20 minutes or until you need it to wrap the pork later.

Heat half the butter in a skillet. Season the pork with sea salt and pepper. Sear all sides of the pork. Cook it about 2/3 thirds of the way. When it's done baking at the end you want the pork to be cooked thoroughly but not dry. Remove from the heat and set aside.

Add more butter to the skillet. Add the mushrooms, apples, sage and garlic. Sauté for several minutes until the mushrooms begin to brown. Pour in the calvados to deglaze the pan. Cook until the liquid is reduced to nearly zero.

On a piece of cling film lay out the prosciutto. Spoon the mushroom and apple mixture onto the prosciutto and spread it out. Coat the pork with the mustard and place the pork in the centre of the apple and mushroom mixture. Tightly wrap the pork in the prosciutto and seal with the cling film. Chill for 30 minutes.

Roll out the pastry thinly. Place the pork into the centre. Trim the sides and wrap the pork like a burrito. Place on a baking sheet and chill for 30 minutes.

Preheat the oven to 425F/220C.

Brush the pastry with a beaten egg. Bake for 30-35 minutes until the pastry is golden and flaky. Let rest for at least 10 minutes before slicing.