Remove the giblets and neck from turkey, and reserve them for Homemade Turkey Stock. Rinse turkey with cold water; pat dry. Trim the excess fat. Sprinkle 4 teaspoons salt over the turkey and in body cavity; refrigerate, uncovered, 24 hours.
Preheat oven to 325°.
Rinse the turkey with cold water; pat dry. Stuff body cavity with onion, celery, herbs, and 1/4 teaspoon pepper. Lift wing tips up and over back; tuck under turkey. Tie legs together with kitchen string. Place turkey, breast side up, on a roasting rack in a roasting pan. Insert a meat thermometer into meaty part of thigh, making sure not to touch bone. Sprinkle turkey with 1/2 teaspoon salt and 1/2 teaspoon pepper. Cover breast with foil. Bake at 325° for 2 hours, basting turkey with 1/3 cup broth every 30 minutes. Remove foil; bake an additional 1 3/4 hours or until thermometer registers 180°, basting turkey with broth every 30 minutes. Remove turkey from oven. Cover turkey loosely with foil, and let stand for 30 minutes before carving. Reserve pan drippings. Discard skin.
Cook the sausage in a large nonstick skillet over medium-high heat until browned, stirring to crumble. Drain and set sausage aside.
Place a zip-top plastic bag in a 2-cup measure. Pour drippings into bag; let stand 10 minutes (fat will rise). Seal bag, and snip 1 bottom corner. Drain into a medium saucepan; stop before fat reaches opening. Reserve 1 tablespoon fat; discard remaining fat. Cook reserved fat and flour in roasting pan over medium heat 1 minute; stir constantly with a whisk. Add drippings, 3 cups broth, and sausage. Combine cornstarch and water; add to pan. Bring to a boil. Cook 2 minutes or until thick; stir constantly.