Allow turkey to stand at room temperature for about an hour.
Preheat oven to 425 degrees with rack in lower third of oven.
Use a serrated knife to cut off top of bread to create a flat surface, if necessary. Cut bread lengthwise, without cutting all the way through. Open bread like a book. Evenly spread butter over cut-side of bread. Place bread, buttered side up, in large, heavy-duty roasting pan.
Liberally season turkey inside and out with salt and pepper. Truss legs together using kitchen twine, and tuck wing tips under the breast. Place turkey, breast side down, on buttered bread. Place in oven, legs first if possible, and roast for 45 minutes. (If it's not possible to position turkey in oven legs first, rotate roasting pan halfway through cooking time.) Remove turkey from oven and reduce temperature to 350 degrees.
Using a wooden spoon and side towels, carefully turn turkey so it's breast side up. Place in oven, breast side first if possible, and continue roasting, turning and basting with pan juices every 15 minutes until an instant-read thermometer inserted into the thickest part of the thigh (avoiding bone) reaches 165 degrees, about 1 1/2 hours more. (If it's not possible to position turkey in oven breast side first, rotate roasting pan halfway through cooking time.) Let turkey rest at least 30 minutes before carving.