Preheat oven to Gas4/180ºC/350ºF. In a saucepan melt butter and oil then fry the onions and garlic until they Colour. Add chicken and cook for a further five minutes until sealed, then add leeks and flour stirring until well combined, followed by wine and chicken stock slowly, until the sauce has thickened.
Next add cream and mustard, then season and remove from heat and stir in prunes, cheese and herbs.
Place pie filling into a medium sized dish. Mix egg yolk and cream, and then brush around the dish. Cover with pastry and trim the sides with a knife, before brushing remaining egg mix over pie. Poke a couple of small holes in the pastry to let out steam.
Cook for 25 minutes. Remove from oven and serve.
Recipe courtesy of the Welsh Assembly Government