Preheat oven to 200°C / 400°F/ Gas 6.
Melt the butter in a large saucepan and sauté the carrots, red onion and mushrooms.
Cook for 5 minutes.
Then stir in the flour using either a wooden spoon or plastic balloon whisk and gradually add the milk, stirring until the sauce thickens, boils and becomes smooth.
In a separate saucepan, bring water to the boil and add your green vegetable and cook for 3 minutes to blanch it. Drain and refresh under cold running water and drain well again.
Add the chicken, vegetable, seasoning and cream to the sauce. Pour the mixture into a 1.25 litre pie dish.
Roll out the pastry to the same size as the dish, dampen the rim of the dish with milk and cover with the pastry. Brush the pastry with beaten egg.
Bake in the oven for 20–25 minutes until the pastry is golden brown.
To serve: Slice on a diagonal and accompany with a mixed salad.