Preheat the oven to 220°C (425°F) and roast all the bones, turning once, until dark brown. Transfer to a stockpot. Cover the bones with chicken stock and cook, covered, for 12 hours over low heat. Strain the stock, return to the stockpot and add the vegetables. Cook over low heat for a further 4 hours.
Strain the stock again and refrigerate until chilled. Remove the layer of fat from the top of the stock and season with salt. Whisk in the xanthan gum to thicken the stock and then strain.