Liberally salt the turkey inside and out; chill in the refrigerator, uncovered, for 12 to 24 hours; rinse under cold water and dry thoroughly.
Preheat the oven to 220°C/425°F/Gas 7. Meanwhile, in a small saucepan over low heat, melt the butter slowly.
Gently run your fingers between the skin and meat on either side of the turkey’s breast bone; arrange the sage leaves between the skin and breast. Pour one-fourth of the melted butter over the turkey and rub it into the skin with your hands. Tuck the drumsticks under the folds of skin or tie together with butcher’s twine. Season the turkey with salt and pepper. Place the turkey on a rack in a roasting pan and insert an ovenproof meat thermometer into the thickest part of the thigh so it points toward the body but does not touch the bone.
Place the turkey in the oven. After 30 minutes, baste the turkey with some of the melted butter and reduce the oven to 175°C/350° F/Gas 4. Continue roasting, basting with the melted butter every 30 minutes until the thermometer registers 70°C/160°F, about 2.5 hours. If the turkey is browning too quickly, cover it with aluminum foil. Adjust cooking time if using a smaller or larger turkey.
Remove the turkey from the oven and transfer to a cutting board; allow it to rest, uncovered, for 30 minutes. Carve and serve with gravy.