Preheat the oven to 180°C/160°C fan/Gas 4.
Heat 2 tablespoons of the duck fat or butter in a large frying pan over high heat and, when hot, add the celery, onion and carrots and cook for 10 minutes, or until softened. Add the garlic and fry for another minute or two. Remove from the pan and set aside in an ovenproof casserole dish.
Combine the flour, salt and pepper, then dust the chicken pieces all over with the seasoned flour. Add the remaining 2 tablespoons of duck fat to the frying pan and fry the chicken pieces in batches until golden brown each batch should take about 10 minutes. Remove from the pan and add to the casserole dish.
Add the bacon, mushrooms and shallots to the pan and fry for 5-7 minutes until well browned, then add to the chicken in the casserole dish.
Deglaze the pan with the brandy then pour this over the ingredients in the casserole dish. Place the casserole dish over the heat, add the wine, bay leaves and thyme (tied together with string) and bring to the boil. Simmer rapidly for about 15 minutes, then add the chicken stock and simmer gently for a further 15 minutes. If needed, mix the cornflour with a little water and stir into the sauce to thicken.
Cover and place in the oven for 40-50 minutes, or until the juices run clear when a chicken thigh is pierced with a knife in the thickest part. Remove the herbs tied with string, stir in the parsley and serve.