Preheat the oven to 160°C/140°C fan/Gas 3.
Place the bacon in a large non-stick ovenproof frying pan or flameproof casserole and fry over a medium heat for a few minutes to render out the fat. Add the onions and continue to fry until the bacon is brown at the edges. Transfer the bacon and onions to a plate using a slotted spoon and set aside.
Lightly season the chicken. Place the oil in the unwashed dish, add the chicken and brown over a high heat, turning once. Transfer to the plate with the bacon and onions.
Scatter the flour into the dish (adding a little more oil, if needed) and stir to combine. Gradually incorporate the hot stock, whisking, and allow to thicken. Pour in the wine and return the bacon, onions and chicken to the pan. Stir well, add the thyme sprigs, sage sprigs and bay leaves. The sauce
will be quite thick at this stage. Bring to the boil, season well with salt and black pepper and cover. Transfer to the oven for 30 minutes.
Remove the dish from the oven and add the mushrooms and creme fraiche. Stir well and return to the oven for a further 15–20 minutes, until the chicken is tender.
Meanwhile, heat the butter in a small pan over a medium heat. Add the whole sage leaves and fry until crisp.
Remove the bay leaves, thyme and sage sprigs from the casserole and discard (some leaves will have fallen off to flavour the casserole). Stir in the parsley and serve with the crisp sage leaves on top.