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Put the mushrooms in a food processor and pulse until they're very finely chopped.
Put half the olive oil in a large frying pan and add the leeks along with a pinch of salt. Fry gently for 15 mins or until softened and golden brown. Scrape the leeks out of the pan and into a bowl. 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 another minute, then leave to cool slightly.
Heat the oven to 200C/180C fan/gas 6. Combine the mushroom mixture with the leeks in a bowl, add the chestnuts, cheese and breadcrumbs. Season lightly to taste (the miso and leeks will add salt, so don’t add too much) and work everything together until you have a slightly stodgy mixture.
Unravel the puff pastry on a floured surface, then roll out so one side measures about 40cm. Push the mushroom mixture down the centre of the pastry. Bring the pastry around the mixture and seal the edges with a fork, then cut into 8 pieces. Lay on a lined baking sheet and brush each one with the beaten egg. Bake in the oven for 25-30 mins until deep, golden brown. Allow to cool a little before eating.