1. Preheat the oven to 200 degrees C.
2. Chop the chicken breast fillets into cubes.
3. Peel and chop the onion and garlic.
4. Rinse mushrooms and chop in half.
5. Heat the butter a saucepan and fry the chicken, onion, garlic, and mushrooms until the onion has gone tranclucent and the chicken is cooked
6.In a seperate saucepan mix the mushroom soup powder and the milk and place this on a medium heat until it comes to the boil.
7. Allow the soup and milk mix to cool and then mix in the parsley, sage, pepper and the fried chicken, mushroom, garlic, onion mix from the first saucepan.
8. Cut about one third from the pastry to set aside for the pie lid.
9. Roll the other larger piece (the other two thirds) out to fit a 5cm deep pie dish. Line the bottom and the sides, tkaing care not to tear the pastry.
10. Fill the pie wit the chicken and mushroom mix.
11. Roll out the smaller one third of the pastry to fit as a lid for the pie.
12. Brush the edges of the bottom pie pastry with the water and then put the lid on top. Press the edges together to close the pie.
13. Beat the egg yolk and the Tbs water and brush over the top of the pie.
14. Make two small holes for steam in the top of the pie.
15. Bake for 30 minutes until the pastry is a golden colour and cooked.