Remove the giblets from the turkey. Slice the liver and set aside (use the remaining giblets for gravy or stock). Pat the turkey dry with kitchen paper then season generously with salt and pepper inside and out. Place the turkey on a rack in a roasting tin.
To make the stuffing, in a frying pan over a medium heat, fry the bacon in a pan for a few minutes to release the fat. Add the onion and mushrooms and cook and stir until softened, about 2 to 3 minutes. Add the chopped liver, chestnuts and fresh parsley. Remove from the heat and transfer to a mixing bowl.
To the mixing bowl add the breadcrumbs, beaten egg and dried sage. Mix well with the hands until well incorporated and the mixture comes together. If the stuffing is too dry add a little more stock, if too wet add some more breadcrumbs. Stuff the turkey with the stuffing mixture then create a tent made out of aluminium foil and cover the turkey (avoiding contact between the foil and the turkey if possible).
Roast the turkey in the preheated oven for 3 to 3 1/2 hours, basting with the cooking juices every 30 minutes or so until the juices from the thickest part of the turkey run clear and the internal temperature reaches 75 degrees C. Remove the foil tent for the last 40 to 60 minutes of cooking to ensure a golden brown skin. Remove from the oven and allow to rest, covered for at least 30 minutes before carving and serving. Spoon out the stuffing and serve with the turkey.