In a large saucepan over medium heat, melt the butter. Add the neck, giblets, onion, and celery. Sprinkle with salt, pepper, and any addtional herbs. Sauté until nicely browned, about 5 minutes.
Add the broth. Bring to a boil and then simmer 30 to 40 minutes. Use a slotted spoon to remove the neck and giblets.
When the chicken or turkey is done cooking, remove to a platter and tent with foil. Pour any pan juices and drippings into the stock, scraping up any browned bits for flavor. Spoon off excess any fat from the top.
Bring to a boil and cook until reduced by about one quarter. Set a fine mesh sieve over a bowl to strain out the solids. Discard the solids and return the liquid to the pan.
Whisk in ½ teaspoon of glucomannan and let sit 10 minutes to thicken. If you like very thick gravy, whisk in the remainging glucomannan. The gravy will continue to thicken up as it cools. If it gets overly thick, add a little chicken broth to thin it out as you re-warm it.
Notes
You can easily scale this recipe up for a larger bird or if you just love to drown everything in gravy. For every cup of liquid, you need about ¼ teaspoon glucomannan.
Storage Instructions
Store the gravy in a glass jar or other covered container for up to 5 days. You can also freeze it for several months. To re-warm, place in a pan and heat over low heat. If you need to thin it out, add more broth a few tablespoons at a time.