Preheat the oven to 200°C.
Fry the chicken in olive oil until it begins to turn white.
Add the mushrooms and fry until the chicken is golden-brown.
Set chicken and mushrooms aside, then soften the onion and garlic in the same pan for 3 minutes.
Melt butter in a saucepan, stir in flour, and cook for 3 minutes to form a smooth paste.
Whisk milk, stock, nutmeg, pepper, and salt into the flour mixture and simmer until thickened.
Stir in parsley, combine with chicken and mushrooms, then spoon into a dish and cool completely.
Roll out the pastry, cover the pie dish, trim the edges, and decorate with pastry leaves.
Brush the pastry with beaten egg, cut steam slits, and bake for 20-25 minutes until golden-brown.