Preheat the oven to 200C , Wrap the chicken breasts individually in tin foil with a tbsp of butter & crush garlic and cook in the oven for 30 mins or until cooked.
While Chicken is cooking - in a saucepan put the shallots, mushrooms and remaining butter. simmer for 5 mins then add in the white wine and cook until the wine has evaporated. remove mixture into a bowel and place in the fridge.
when chicken is cooked remove from the foil but spread the mustard over then place in the fridge for 30 mins. (as for the mixture above) Remove pastry from fridge to bring to room temp.
roll out the pastry - do a small layer of your onion & mushroom mixture. Coat chicken breast in the garlic and herb phili then sit the chicken on top - put a layer of mixture on top and fold the pastry to make a parcel.
cover the pastry with the beaten egg yolk and make silts in the top pastry for air to get out.c
cook on 200C for 10 mins then reduce heat to 180C and cook for another 15 mins.