Put the bones and meat back in the pan and cover with three litres of cold water, or as much as you can fit in. Add the onion and ginger, the spices and orange peel (preferably in a muslin pouch or similar to make life easier later) and bring to the boil. Then turn down the heat and simmer gently for at least five hours, skimming as necessary, until the oxtail is falling off the bone. Strain, retaining the oxtail and, if you have time, cool the broth and skim the fat from the top if desired. You should have about two litres.