Preheat the oven to 160°C (140°C Fan/Gas 3).
Cook the chicken with celery, bay leaves, wine, and stock until tender. Remove the meat and strain the cooking liquid.
Fry the bacon, onion, garlic, and mushrooms until softened.
Add butter and flour to make a roux, then gradually whisk in the reserved stock until thick.
Stir in cream, mustard, parsley, seasoning, and chicken.
Spoon the filling into a greased pie dish and cool completely.
Cover with puff pastry or create a lattice top. Brush with beaten egg.
Bake at 200°C (180°C Fan/Gas 6) for 30–35 minutes, until golden brown and bubbling.
Rest for 5–10 minutes before serving.