Put the mushrooms into a food processor and pulse until they are very finely chopped. Put half the olive oil in a large frying pan, add the leeks along with a pinch of salt and fry gently for 15 mins or until softened and golden brown. Remove the leeks from the pan, put them into a bowl, and set them aside to cool a little.

Heat the remaining oil in the pan and fry the mushrooms for 10 mins over a medium heat. Add the garlic, sage, miso and mustard, and fry for a further minute. Leave to cool slightly.

Heat the oven to 200C/180C fan/gas 6. Tip the mushroom mixture into the bowl with the leeks, then add the chestnuts and breadcrumbs. Season, then mix everything together

Roll the pastry out so that one side measures 43 cm. Mould the mushroom and leek mixture into a sausage shape down the centre of the pastry, then bring the pastry up around the filling and seal along the seam with a fork. Cut into ten pieces. Lay on a parchment-lined baking sheet and brush each piece with milk. Bake for 25 mins or until deep, golden brown. Leave to cool a little and sprinkle with sesame seeds before serving.