Whisk together all the marinade ingredients. Pour into a shallow dish.
With a sharp knife, score parallel lines in the duck skin and fat – take care not to cut through to the flesh. Put in the marinade, skin-side up, to cover the meat while leaving the skin marinade-free. Chill, uncovered, for at least 2 hours.
Pre-heat the oven to 200°C/Fan 180°C/Gas 6.
Pat the skin with kitchen roll, to remove any traces of marinade. Put in a cold, non-stick frying pan, skin side down, then turn on the heat to medium. Cook for 5 minutes, until lots of fat runs out and the skin is golden and starting to crisp. Drain off the fat and reserve – it’s great for roasting vegetables. Turn the duck over and cook for 1 minute.
Transfer the duck to a roasting tin, skin side up. Cook in the oven for 10 minutes if you like your meat slightly pink, or 12 minutes for well done. Put on a warm plate and cover with foil. Rest for 10 minutes.
Meanwhile, strain the marinade into a pan, heat until boiling, then simmer for 5 minutes. Mix the cornflour with 3 tbsp cold water and stir into the marinade. Simmer for another 2 minutes. Pour on the duck.
Serve with the rice and veg.