Add the onions and mushrooms, then fry on a high heat for another 3 mins until the onions start to brown.
Add the garlic and cook another minute.
Add the flour into the pan and cook, stirring, for 1 min.
Remove the pan from the heat, gradually stir in the stock, followed by the milk, then add the chicken and parsley.
Bring to the boil, then simmer for 5 minutes.
Spoon the filling into a baking dish.
On a floured surface, unroll the pastry and cut along the pie dish leaving about an inch over the size of the dish.
Cover the filling in the dish with the grated cheese.
Cove the filling with the cut dough. Using a little of the egg, fix to the edge of the pie dish. Brush with egg, then lift the rest of the pastry over the pie. Gently roll the extra over and press the edges with your fingers. Brush lightly with egg to glaze, prick with a fork, and then bake for 25-30 mins or until the pastry is risen and dark golden brown.