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Put oven rack in lowest position and preheat oven to 425°F.
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Tie celery, parsley, thyme, marjoram, sage, and bay leaf into a bundle with kitchen string to make a bouquet garni.
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Rinse turkey inside and out and pat dry. Rub turkey inside and out with salt and pepper, then put onion and bouquet garni in large cavity. Working from large cavity end, run your fingers between skin and flesh of breast to loosen skin without tearing. Put 1 tablespoon butter under skin of each side of breast and massage skin from outside to spread butter evenly. Tie drumsticks together with kitchen string and fold wings under body. Put turkey on rack in a large flameproof roasting pan and, if using remote thermometer, insert it into thickest part of a thigh (do not touch bone).
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Brush remaining 2 tablespoons butter over turkey, then roast 30 minutes.
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Reduce oven temperature to 350°F. Baste turkey with pan drippings or butter, then continue to roast, basting every 30 minutes, until a thigh registers 165°F on thermometer, 2 to 2 1/2 hours more.
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Carefully tilt turkey so any juices from inside cavity run into roasting pan, then transfer turkey to a serving platter and discard onion and bouquet garni from cavity. Let turkey stand 30 to 40 minutes (thigh temperature will rise to 175°F).