In a pot place the chicken carcass, onion, carrot, celery, bay leaves, peppercorns and parsley into a pot. Just barely cover with water and boil for at least 2 hours, or simmer for 8 hours if you have time.Strain and reserve the stock.
Make the filling
In a large pan, melt the butter over medium heat.
Add the leeks, celery leaves and thyme and cook slowly until soft and sweet, about 10–15 minutes. Add the garlic and cook for another minute.
Stir in the flour and cook for 2 minutes.
Gradually add the chicken stock, stirring continuously until smooth and thickened.
Add the cream, Dijon mustard and a few splashes of Maggi seasoning.
Blend the chicken skin
Crisp the reserved chicken skin in a pan or oven until deeply golden.
Blend the crispy skin with a ladle of the sauce until it forms a rich paste.
Stir this back into the filling. It adds massive flavour, richness and body.
Season well with salt and black pepper.
Fold through the chicken meat, parsley and lemon zest.
Notes
The filling should be rich, glossy and deeply savoury with a fresh lift from the lemon.