Heat a large shallow saucepan with the butter and oil - to stop the butter from burning.
Add in the chicken and ham and cook for 4-5 minutes.
Add in the leeks, onions, garlic and mushrooms at this stage together with a little seasoning.
After cooking for a further 5 minutes sprinkle in the flour and stir to thicken the sauce.
Pour in the white wine, cheese 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 at this stage too.
Simmer for 5-6 minutes and then transfer to a large casserole dish.
Allow to cool for 15 – 20 minutes before adding the layer of pastry.
Neatly arrange the pastry on top.
Brush the pastry lightly with a little egg wash and bake in a preheated oven (190C/375F/Gas Mark 5) for approximately 30 minutes.