Preheat oven to 180°C, 350°F, Gas Mark 4, 10 minutes before cooking. If using a frozen turkey, ensure it is thoroughly thawed and no ice crystals remain in the body cavity. Remove giblets if necessary. (Use giblets to make stock for gravy). Wipe the turkey inside and out with either a clean damp cloth or kitchen paper. Pat dry.
Using your fingers, carefully lift the skin away from the flesh of the turkey breast, easing the skin by gently placing your forefinger at one side of the neck and pushing upwards. Do a little at a time on each side until the skin is free. Once free, gently smear the prepared stuffing over the flesh then cover with the skin and secure the neck flap with either sewing with fine twine and a trussing needle or skewers.
Season the body cavity then place the onion and fresh herbs inside. Weigh the turkey and calculate the cooking time allowing 20 minutes to the kg plus 90 minutes.
Place in a roasting tin and spread the breast and thighs with the softened butter and place the bacon over the top. Cover loosely with foil and cook in the preheated oven for calculated cooking time. Remove foil for the last 40 minutes of the cooking time.
Continue to cook for the remaining time, basting occasionally with the turkey juices, until cooked and the juices run clear when the thigh is pierced with a skewer. If cooking without foil, turn the turkey over so it is the right way up, again basting occasionally until cooked. (Try to avoid opening the oven door too often during cooking). Add a little of the pan juices to the gravy.
Transfer to a platter, cover loosely with foil or a clean tea towel and leave to rest for at least 15 minutes before carving. Serve with roasted potatoes and parsnips, sprouts and carrots.