Make the filling. Place the chicken in a large pan.
Chop the onions and garlic and add them with the bay leaves.
Season with salt and pepper.
Add 500ml milk, the stock cube and add water until the chicken is covered.
Bring to the boil and simmer for 30 minutes.
Remove chicken and leave to cool.
Strain the cooking liquid and keep 500ml.
In a saute pan add the butter and water.
Chop the mushrooms and leeks and cook in the pan until soft
Add the flour and stir until a paste is formed then slowly add the milk and reserved chicken liquid stirring until a sauce is formed.
Bring to the boil and stir until thick.
Take the skin off the chicken and chop into bits.
Add the chicken to the sauce.
Chop the parsley and add it.
Pour the filling into a large ovenproof dish.
Preheat the oven to 200C Gas 6
Flour and board and roll out the pastry.
Cover the dish and make some holes to let the air out.
Brush with milk.
Cook for 30-35 minutes until golden and bubbling