Put the gammon, onion, tangerine zest and star anise in a large pan. Pour over all but 100ml of the ginger beer and, if necessary, top up with water so the gammon is just covered. Bring to the boil, skim the surface to remove any fat, reduce to a simmer, then cover and cook for 3-3½ hrs or until the meat is tender. When the gammon is cooked, remove it from the pan. Reserve the cooking liquor and use to cook lentils (see 'Goes well with'). Cool, cover and chill at this stage if you want to prepare ahead – bring back to room temp before continuing.