Heat oven to 375° F. Working on a baking sheet, remove the giblets and neck from the cavity. Reserve the neck and discard the giblets. Using a paper towel, pat the turkey dry of juices. Stuff with the thyme and half the onions.
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Tie the legs together with kitchen twine. Tuck the wing tips underneath the body (this will prevent them from burning). Watch a video to learn how to tuck turkey wings.
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Rub the turkey all over with the butter and season with 1 teaspoon salt, making sure to reach the crevices of the legs and wings. Be careful not to tear the delicate skin.
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Place the turkey neck, carrots, celery, and remaining onions in a large flameproof roasting pan. Put a roasting rack in the pan and place the turkey on top of it.
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Roast the turkey, basting every 30 minutes with the pan juices. (Basting will not make the meat moister, but it will produce an evenly browned skin.)
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If the turkey begins to darken and there is still a substantial amount of cooking time left, tent it loosely with foil. If the vegetables begin to scorch, add some broth to the pan.
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Continue roasting until the thickest part of a thigh registers 165° F, 2½ to 3 hours (watch this video on taking a turkey's temperature for the complete how-to). Tilt the turkey to empty the juices into the pan. Transfer the turkey to a carving board, tent with foil, and let rest for at least 25 minutes. Reserve the pan and its contents for Basic Gravy.