1 sheet puff pastry
1 egg beaten with a little milk
heat a large shallow saucepan with a oil.
Add in the chicken and cook for 4-5 minutes. Add in the leeks and mushrooms at this stage together with a little seasoning.
After a further 5 minutes sprinkle in the flour and use this to dry up any liquids in the pot (and thicken the sauce), Pour in the white wine and the milk at this stage and allow the mixture to come to a gentle boil whilst stirring all the time. If you like you could add a little cream as well. Season lightly and add the wholegrain mustard at this stage also.
Simmer for 5-6 minutes and then transfer to a large casserole dish.
Neatly arrange the sheet of puff pastry on top, brush with the egg wash and if desired arrange some leaves cut from puff pastry on the topand bake in a preheated oven (190C/375F/Gas Mark 5) for approximately 20-25 minutes.