Preheat oven to 350°F. Position a rack in a large roasting pan.
Rinse turkey inside and out; pat dry. Place turkey, breast side up, on rack in roasting pan. Season inside and out with salt and pepper. Tuck under skin at neck end, fold wings under and, using kitchen string, tie legs together.
Roast turkey for 2 1/4 hours. When pan juices begin forming (check after first hour) baste with juices every 30 minutes.
Whisk together mustard and molasses.
Remove turkey from oven and brush with molasses mixture. Return to oven and continue roasting about 45 minutes longer, basting every 10 to 15 minutes (tent bird lightly with foil if it begins to get too dark), or until a meat thermometer inserted in thickest part of thigh, away from bone, registers 165°F.
Remove string from legs and let turkey stand, loosely covered with foil, for 20 minutes to let juices set. Carve turkey and arrange on a platter. Serve warm with your favorite gravy.