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, salt, pepper, and and parsley. Bring to a boil, lower the heat to low, and simmer for 5 minutes to thicken. Taste for seasoning and add more salt as needed.
Spread the filling in a 10-inch (round or square) ovenproof dish and chill for 30 minutes.
If necessary, roll out your pastry to cover the top of your pan, leaving about an inch of extra dough. Top the filling with cheddar cheese, cover with puff pastry, and press down firmly to seal and crimp the edges. Then, finish by brushing with the beaten egg.
Prick the top of the pastry with a fork (or make slits with a knife), and then bake for 25 to 30 mins or until the pastry is risen and dark golden brown.
Allow to rest for 10 minutes before serving.
Video
Notes
For even cooking, rotate the pie halfway through baking.
To prevent soggy pastry, chill the assembled pie for 10 minutes before baking.
Add a dash of nutmeg to the sauce for a subtle warmth.