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Heat oven to 200C/180C fan/gas 6. Fry the sausages in a large frying pan until browned all over, remove and set aside. Remove most of the fat left in the pan, then add leeks and soften for 8-10 mins. Add the peeled apple slices and cook for 1 min more. Stir in the flour, followed by the mustard. Pour over the stock and bubble for 2 mins to make a sauce.
Chop the sausages into chunks and stir into the pan. Then, tip everything into a large ovenproof dish. Top with the pastry, trimming off any excess and glaze with the egg. Bake for 25 mins until golden.