Preheat oven to 450°.
Remove and discard giblets and neck from turkey. Trim excess fat. Combine onion, carrot, celery, rosemary, sage, thyme, 1/2 teaspoon salt, and 1/2 teaspoon pepper in bowl. Stuff body cavity with onion mixture. Tie legs together with kitchen string. Lift wing tips up and over back; tuck under turkey. Sprinkle turkey with remaining 1/2 teaspoon salt and remaining 1/2 teaspoon pepper. Place turkey, breast side up, on a roasting rack in a roasting pan; pour 2 cups water into pan.
Bake at 450° for 1 hour and 30 minutes or until a thermometer inserted in meaty part of thigh registers 165°. Remove turkey from oven; let stand at least 30 minutes before carving. Discard onion mixture and skin.
Place a zip-top plastic bag inside a 2-cup glass measure. Pour drippings into bag; let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner. Drain drippings into a bowl, stopping before fat layer reaches opening; discard fat.
Place the roasting pan over two burners on medium heat. Add the drippings and Make-Ahead Gravy, scraping the pan to loosen browned bits. Reduce heat to medium-low; cook until gravy mixture is reduced to 3 cups, stirring occasionally (about 5 minutes).