Place the chicken, shallots, garlic, celery, rosemary and black peppercorns in a large saucepan. Add the balsamic vinegar and pour in enough cold water to almost cover the chicken. Place over a medium heat and bring slowly to the boil, skimming the surface to remove any scummy froth. Cover and simmer gently for 1 hour.
Add the preserved lemon and half the sage leaves, then simmer gently for a further 15–30 minutes, depending on the size of the chicken, until the juices run clear when the thickest part of the leg is pierced with a knife. Carefully remove from the pan and place in a deep dish, cover with foil and leave to rest. Increase the heat and boil the stock for 20–25 minutes or until reduced by half. Remove from the heat and leave to cool slightly. Season to taste.
Heat the oil in a small frying pan and shallow-fry the remaining sage leaves for 30 seconds until crisp. Remove with a slotted spoon and drain on kitchen paper.
Cut the chicken meat from the carcass, discarding the skin, and spoon into shallow bowls with plenty of cooking broth. Garnish with the crisp sage leaves and serve with steamed asparagus and broccoli.