Remove giblets and neck from turkey, and discard. Rinse turkey with cold water; pat dry. Sprinkle turkey inside and out with salt and pepper. Place turkey in a deep roasting pan; pour buttermilk over top. Cover and chill 8 hours, turning several times.
Drain turkey, discarding marinade; place turkey, breast side up, in a shallow roasting pan; tuck wing tips under. Combine 1 cup sherry and next 4 ingredients; spoon into cavity. Close cavity with skewers, and truss. Brush turkey with melted butter. Insert a meat thermometer into meaty part of thigh.
Bake at 350° for 4 hours or until meat thermometer registers 180°, basting with remaining sherry every 15 minutes. Remove turkey; reserve 1/4 cup drippings. Discard stuffing.
Heat reserved 1/4 cup drippings in a heavy saucepan; add flour, stirring until smooth. Cook, stirring constantly, 1 minute. Gradually add milk; cook over medium heat, stirring until mixture is thickened. Salt and pepper to taste.