Preheat oven to 275 degrees.
Pat the turkey dry and place it on a roasting rack, breast side up. Cross the legs and tie them together with kitchen twine.
Cover the whole pan with heavy aluminum foil, tucking it underneath the pan.
Roast for the first stage for about 10 minutes per pound (so, for a 20 pound turkey, about 3 1/2 hours.)
Using a vegetable peeler, shave off thick slices of orange peel and slice them very thin.
Combine with butter, rosemary, salt and pepper.
After the first stage of cooking, remove the turkey from the oven and remove the foil (turkey will still be pale.)
Smear the butter mixture all over the skin, in crevices, etc so that it's totally covered.
Increase the oven temperature to 350 degrees and return the turkey to the oven, basting every 30 minutes.
Continue roasting the turkey until the thermometer reads 165 to 168 (about 2 1/2 more hours), then remove the turkey from the oven.
Carve and serve.