1. Preheat the oven to 210ºC.
2. To make the filling, dice the chicken and prepare all vegetables.
3. Heat butter and oil in a large, deep frying pan. Add chicken and stir for 2-3 minutes (its ok if it is a little undercooked). Add sliced leek, spring onions and cook for a further few minutes until leek is soft.
4. Add flour and stir in well. Add mushrooms, Dijion mustard, stock, thyme leaves and season with salt and pepper. Allow mixture to thicken slightly and reduce liquid by half. Add cream and stir well. Turn off heat.
5. Place mixture into a pie or oven proof dish and cover the top with puff pastry. Cut a few air vents/slits in pie. Brush pastry with lightly beaten egg and cook for 20-25 minutes until pastry is golden brown. Serve.
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