Chop the celery, onion, carrot and garlic.
Remove the giblets and neck from the cavity of the turkey, they should be in a little bag, and rinse them out.
Melt the butter in a stock pot and add the giblets and neck. Brown on all sides for a few minutes.
Add the chopped celery, carrot, onion, garlic and the 8 cups (67 fl oz) of water. Put on medium-low heat and simmer for 60 minutes, or until you have exactly 3 cups (25 fl oz) of broth left. Turn off the heat.
Strain the veggies out and discard. Carefully remove the neck and giblets out. You should have 3 cups (25 fl oz) of broth in the stock pot.
Let the neck cool down a bit before doing this step or you'll burn your fingers. Peel the meat out of the neck as much as you can. Once you've peeled all of the neck, add it to a food processor along with the giblets.
Pulse until you get a thick paste.
In the stock pot, add the 2 cups (17 fl oz) of roast drippings. A 14lb/6.57kg turkey gave me 2 cups (17 fl oz) of juice. Add the giblet and neck paste into the pot, turn on the heat and start stirring.
Mix the tapioca starch and water together and pour it into the stock pot.
Continue whisking until the gravy has considerably thickened. Once thickened, add the parsley and it's ready to serve!