I attempted to defrost it but it was still pretty frozen when it went in the oven.
Preheat oven to 325 degrees.
After we rinsed and patted dry the 18 lb. turkey, we place it in a roast pan on a rack, breast side up. We quartered 4 medium onions, putting half of the chunks in the cavity of the turkey and half around the turkey in the pan. We also cut up two large stalks of celery and 2 large carrots for the pan (make sure the neck and giblets are in the bottom of the pan too). We put 6 sprigs of fresh rosemary in the cavity and rubbed the entire turkey with salt, pepper and 2 teaspoons dried thyme. We poured 1 cup of water in the bottom of the pan. Covered and baked the bird for 3 hour 1/2 hours. Uncovered, basted and cooked for 1 1/2 more hours. Basted periodically (you can add more water if the bottom of the pan dried out).Save the pan drippings to make the gravy. Add some sliced mushrooms to the drippings and let simmer for 8 minutes before you add the thickening agent.