Preheat the slow cooker if necessary; see the manufacturer's instructions. Dry-fry the duck in a large frying pan over a low heat until the fat begins to run, then increase the heat until the duck is browned on both sides. Lift out of the pan with a slotted spoon and transfer to the slow cooker pot.
Pour off any excess fat to leave about 1 tablespoon. Fry the onions until softened. Stir in the flour, then mix in the stock, wine, orange juice, bay leaf, a little salt and the crushed peppercorns and bring to the boil, stirring. Add the sliced orange.
Pour the sauce over the duck, cover with the lid and cook on high for 4–5 hours or until the duck is tender and almost falling off the bones. Serve with rice and steamed green beans.