Preheat the slow cooker if necessary; see the manufacturer's instructions. Dry-fry the duck legs in a frying pan over a low heat at first until the fat begins to run, then increase the heat and brown on both sides. Lift out of the pan with a slotted spoon and transfer to the slow cooker pot.
Pour off all but 1 tablespoon of the duck fat from the pan, then add the onion and fry, stirring, for 5 minutes or until just turning golden. Stir in the flour, then gradually mix in the stock. Add the remaining ingredients, except for the plums, and bring to the boil, stirring.
Pour the sauce over the duck, add the plums and press the duck beneath the surface of the liquid. Cover with the lid and cook on high for 5–6 hours or until the duck is almost falling off the bones. Serve with rice or with gingered noodles (see below).