To make the stuffing, heat the oil in a large frying pan over medium heat. Add the onion, pancetta and pine nuts and cook, stirring, for 5 mins or until onion softens. Add the garlic and rosemary. Cook, stirring, for 2 mins or until fragrant. Remove from heat. Stir in breadcrumbs and parsley. Season with pepper.
Preheat oven to 180°C. Remove and discard the turkey neck. Rinse the turkey, inside and out, under cold running water. Pat dry, inside and out, with paper towel. Spoon the stuffing into the cavity. Tie the legs together with kitchen string and tuck the wings under the turkey.
Place the turkey on a greased wire rack in a roasting pan. Pour 2 cups (500ml) water into the base of the pan. Brush the turkey with the butter. Cover the turkey and pan tightly with greased foil and roast for 1 hour. Remove the foil and return to oven. Baste the turkey with pan juices every 20 mins and add more water to the pan if necessary, for a further 1 hour and 40 mins or until golden and juices run clear when thigh is pierced with a skewer. Cover loosely with foil and set aside to rest for 20 mins.
Meanwhile, to make the relish, heat the oil in a medium saucepan over medium heat. Cook the onion, stirring, for 5 mins or until soft. Add the apple, cranberries, cinnamon, sherry or port, sugar and vinegar. Bring to the boil. Reduce heat to medium-low and simmer, uncovered, for 15 mins or until mixture has thickened. Serve relish warm or at room temperature.
Place the turkey on a serving platter and serve with the relish.