1. Preheat oven to 425°.
2. Remove giblets and neck from turkey; reserve for Homemade Turkey Broth. Pat turkey dry. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Lift wing tips up and over back; tuck under turkey.
3. Combine butter, chopped sage, thyme, 1 1/4 teaspoons salt, marjoram, and 1/2 teaspoon pepper; rub mixture under loosened skin and over the breast and drumsticks. Place onion, celery, and sage leaves in turkey cavity; tie legs together with kitchen string. Let stand at room temperature for 30 minutes. Place turkey, breast side up, on the rack of a roasting pan coated with cooking spray. Pour 2 cups Homemade Turkey Broth in bottom of pan; place rack in pan. Bake turkey at 425° for 30 minutes. Reduce heat to 325° (do not remove turkey from oven). Bake at 325° for an additional 1 hour and 45 minutes or until a thermometer inserted into meaty part of thigh registers 165°. Remove turkey from pan; cover loosely with foil. Let turkey stand 20 minutes; discard skin.
4. Place a large zip-top plastic bag inside a 4-cup glass measure. Pour drippings through a fine sieve into bag; let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain drippings into a medium bowl, stopping before fat layer reaches opening; discard fat. Add enough of remaining 2 cups Homemade Turkey Broth to drippings to equal 3 cups. Combine cornstarch and 3 tablespoons water in a small bowl, stirring with a whisk. Bring broth mixture to a boil; stir in cornstarch mixture. Bring to a boil; reduce heat, and simmer 5 minutes or until thickened, stirring frequently. Stir in remaining 1/2 teaspoon salt and remaining 1/4 teaspoon pepper. Serve gravy with turkey.