Preheat oven to 150C. Bring stock and spices to the simmer in a saucepan over medium heat, season with sugar and a pinch of sea salt.
Meanwhile, place duck halves, skin side down, in a roasting pan to fit snugly. Scatter over onion, carrot, celery and garlic. Add stock, cover with foil, roast until meat is very tender (2½ hours). Cool duck in stock, then remove wishbone and ribcage bones (discard), set duck aside and keep warm. Strain stock into a saucepan (discard vegetables and spices) and cook over medium heat until reduced by half (20-25 minutes).
Meanwhile, for Savoy cabbage, heat duck fat in a large saucepan over medium heat, add pancetta, cook until slightly rendered and crisp (8-10 minutes). Add cabbage and garlic, cook on low heat, tossing occasionally, until cabbage is tender (15 minutes). Remove from pan, season to taste and keep warm.
Increase oven to 220C. Place duck halves, skin side up, in a flameproof roasting pan. Brush with olive oil, season to taste, then add enough reserved stock to surround but not cover the duck (about 300ml; remaining can be reserved for another use). Roast until duck skin is slightly crisp and golden (10-15 minutes), transfer to a plate and keep warm.
Meanwhile, bring braising liquid to the simmer over medium heat, add blood plum vinegar to taste.
Separate duck breasts from legs, carve breasts into four pieces and legs into two and arrange on a plate. Spoon over braising liquid and serve with cabbage.
Note Blood plum vinegar is available from select delicatessens. If unavailable, substitute raspberry vinegar.
This recipe is from the October 2010 issue of Australian Gourmet Traveller.