Preheat oven to 180ºC / 350ºF. In a medium sized saucepan melt the butter and oil then fry the onions and garlic until they start to colour, about 10 minutes. Then add the chicken and cook for a further 5 minutes until the chicken is sealed. Add the leeks and flour stirring continually until they are well combined. Add the wine and chicken stock slowly until the sauce has thickened. Add the cream and mustard and then season. Take off the heat and stir in the prunes, cheese and herbs. Place the pie filling into a medium sized pie dish. Mix the egg yolk and cream together and then brush around the pie dish. Cover with pastry and cut around the sides with a knife. Brush the remaining egg mix over the pie. Poke a couple of small holes in the pastry to let out the steam. Cook in the oven for 25 minutes, remove from the oven and serve.