Place the gammon in a large saucepan, cover it with water and bring to the boil, then drain and rinse any foam from the joint. Clean the pan.
Return the gammon to the pan and add the cider. Push two cloves into each onion half, then add them to the pan with the bouquet garni, carrots, celery, juniper berries and peppercorns. Pour in enough water to cover the joint and bring to the boil, then lower the heat and simmer the gammon for 1 hour 15 minutes. Take care not to cook it any longer or it will become tough and dry.
Remove the gammon from the pan, discard the cooking liquid and the flavourings, and leave the joint to stand for 15 minutes.
Trim away any fat from the joint, then carve the meat into slices and serve.
Variation: to serve the joint cold, leave it to cool in the cooking liquid before slicing it. This keeps the meat succulent.