Unroll turkey on a large cutting board. With a sharp knife, remove the wings (save for another use). Flatten turkey to 3/4-in. thickness. Cut between the turkey breast and thighs to separate into three sections.
In a large skillet, saute celery and onion in butter until tender. In a large bowl, combine bread and cornbread; add celery mixture, 1/2 teaspoon salt and sage. Stir in enough broth to moisten.
Spoon 2 cups stuffing over turkey breast to within 1 in. of edges. Roll up jelly-roll style, starting with a long side. Tie with kitchen string at 1-1/2-in. intervals. Place on a rack in a large shallow roasting pan.
Spoon 1 cup stuffing over each thigh section to within 1 in. of edges. Roll up jelly-roll style, starting with a short side. Tie with kitchen string; place on a rack in another shallow roasting pan.
Brush turkey with oil; sprinkle with pepper and remaining salt. Bake turkey breast at 325° for 2 to 2-1/2 hours or until a thermometer reads 170°; bake thighs for 1-1/2 to 1-3/4 hours or until a thermometer reads 180°. Stuffing should reach 165°. Let turkey stand for 15 minutes before slicing.
Editor's Note: Ask the butcher to debone the turkey for you.