Serve this with Curtis's Leek and Celery Stuffing
Preheat oven to 180C (350F).
Mix the butter, thyme, parsley and orange zest ingredients in a medium bowl. Season the butter mixture to taste with salt and pepper.
Beginning at the neck end of the turkey and using your fingers, carefully make a pocket between the flesh and skin of the turkey breasts.
Gently slide all but 1 tablespoon of the butter mixture into the pocket under the skin, and spread the butter to cover the turkey breasts.
Slide the orange slices under the skin and over the breasts.
Stuff the main turkey cavity with the garlic halves, thyme sprigs, and a quarter of the onions, carrots and celery.
Truss the turkey then spread the reserved 1 tablespoon of butter mixture over the turkey. Sprinkle with salt and pepper.
Place the remaining onions, carrots, and celery in the bottom of a heavy large roasting pan.
Place the turkey on top of the vegetables and pour over 1 cup of broth. Cover the breasts with foil. Roast for 2 hours. Remove the foil covering the turkey breasts and add the remaining 1 cup of broth to the pan.
Continue roasting until an instant-read meat thermometer inserted into the thigh registers 80C, about 3 1/2 to 4 hours. Remove from the oven and let rest 30 minutes.