For the stock: In a medium saucepot, add enough water (approximately 5 cups) to cover the turkey neck, giblets, carrot, celery, and onion. Bring the water to a boil; reduce heat and simmer on low, skimming the top occasionally for any foam. Cook the stock for at least one hour and up to two hours. Discard the turkey pieces and vegetables and strain the stock through a cheesecloth-lined strainer into another bowl. Strain again, if desired.
For the gravy: In a wide pot or skillet (just deep enough to accommodate the stock), melt the butter over medium heat. Add the onions and saute until tender, about 5 minutes. Add the bourbon and cook until nearly evaporated. Add the turkey stock and cornstarch and whisk vigorously until smooth. (Note: you can supplement with good-quality chicken stock if necessary.)
Increase the heat to high and bring the gravy to a boil. Cook, stirring frequently, until it has reduced down and is thick enough to coat the back of a spoon, 15 to 20 minutes. Season to taste with salt and pepper. Stir in the sage and cook for one more minute. Adjust seasoning, if necessary. Turn off heat and keep warm until ready to serve.