In a medium saucepan bring stock, gizzards (optional), onions and garlic to a boil. Reduce heat to medium and simmer for 20-30 minutes, until onions are soft.
Add drippings from roasting pan, simmer 5 minutes. Strain out solids and transfer broth, onion and garlic to a blender ; dispose of gizzards.
Purée on low, then work up to high speed, blending until mixture is smooth. Transfer mixture to a skillet and reheat gravy until boiling.
Meanwhile, in a small dish, mix kuzu with ¼ cup water, stirring until completely dissolved.
Stir kuzu into gravy until thoroughly combined and gravy begins to thicken, reduce heat. Simmer gravy and reduce to desired thickness, anywhere from 10 to 30 minutes; add salt and serve!