Pre-heat oven to 180oC/160oC Fan/Gas Mark 4.
Prepare the butter by mixing the lemon and clementine zest into the softened butter. Add a few twists of black pepper.
To prepare the turkey, using scrubbed hands, gradually release the skin from the meat. To do this start at the neck end (where the flap of skin is) and carefully push your fingers under the flap and gradually loosen the skin, taking care not to break the skin. You should be able to slowly work your way over the entire breast. Next, take small nuggets of citrus butter and work them over the breast meat under the loosened skin. Finally, take individual fresh sage leaves and arrange under the skin, smoothing back the skin as you work.
Removing the wishbone makes carving easier so do this now, if desired, by sliding a small knife along either side of the bone (you should be able to feel an upside down V shape) and then twist out.
Halve the zested lemon and clementine and place in the large cavity along with the onion wedges.
To prepare the Stuffing, heat the butter and oil and cook the chopped onion until softened but not coloured. Remove from the heat and stir in the breadcrumbs and chopped sage. Use this mixture to stuff the neck end of the turkey and then tuck the flap of skin under the turkey to enclose the stuffing. Any leftover stuffing can be rolled into balls and cooked separately.
Weigh the turkey and calculate the cooking time: 40 mins per kg.
Place the turkey in a roasting tin, rub all over with olive oil and sprinkle with salt. Cover with a tent of foil. Roast according to the calculated cooking time.
Thirty minutes before the end of cooking, remove the foil tent and increase the oven temperature to 200oC/180oC Fan/Gas Mark 6. Continue to cook the turkey for 30 more minutes until golden. To test, insert a skewer into the thickest part of the thigh and the juices should run clear, not pink. If still a little pink, return to the oven and check every 10 minutes until clear.