1. Rinse giblets and neck; chill liver airtight to add later, or save for other uses. In a 5- to 6-quart pan, combine giblets, neck, onions, carrots, celery, and 1 cup water. Cover and bring to a boil over high heat, then reduce heat and simmer for 15 minutes. Uncover; stir often over high heat until liquid is evaporated and giblets and vegetables are browned and begin to stick to pan, 15 to 20 minutes longer.
2. Add 1 quart broth and the pepper to pan; stir to scrape browned bits free. Cover, reduce heat, and simmer, stirring occasionally, until gizzard is tender when pierced, 1 to 1 1/2 hours. If desired, add liver and cook 10 minutes longer.
3. Pour mixture through a fine strainer into a bowl. If desired, reserve neck, giblets, and liver for gravy; pull meat off neck and finely chop neck meat, giblets, and liver. Discard bones and vegetables. Measure turkey stock; if necessary, add more chicken broth to make 1 quart.
4. In the unwashed 5- to 6-quart pan, combine the 1 quart turkey stock and chopped neck meat, giblets, and liver, if using.
5. When turkey is done, remove the rack and the bird from roasting pan. Skim off and discard fat from pan juices. Add 2 more cups chicken broth to roasting pan and stir over low heat to scrape browned bits free. Pour mixture, through a fine strainer if desired, into turkey stock and bring to a boil over high heat.
6. In a small bowl, blend cornstarch with 1/2 cup water until smooth. Add to stock mixture and whisk until boiling, 3 to 5 minutes. Add salt to taste.
You can prepare through step 4 up to 1 day ahead; cover and chill. After turkey is done, continue with step 5.