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Heat oven to 160C/140C fan/gas 3. Put the duck on a rack set over a roasting tin and prick the skin all over with a skewer. Season well. Halve one of the oranges and squeeze some of the juice into the duck cavity and the tin, then stuff the cavity with both of the halves. Pour 100ml water into the tin and roast for 20 mins per 500g (about 50 mins). Turn the oven up to 220C/200C fan/ gas 7 and roast for another 20 mins until the duck skin is crispy and golden.
Put a small pan of water on to boil. Using a peeler, pare the zest from the remaining orange. Scrape any pith off the zest using a small, sharp knife. Add the zest to the water and gently boil for 8-10 mins, then drain, slice thinly into strips and set aside. In the same pan, dissolve the brown sugar with the balsamic vinegar. Cut the pared orange in half, juice it and pour the juice into the pan. Add the Grand Marnier and 1 tsp butter. Bubble until it has reduced by half and is dark and syrupy. Remove from the heat and stir in the orange zest.
Once the duck is cooked, wrap it in foil to keep warm and put it on a board to rest. Spoon off the fat from the roasting tin and discard. Pour any remaining duck juices into a wide, shallow saucepan. Tip in the stock and bring to the boil. Add the wine and simmer for 2-3 mins, then add half the orange sauce and stir well. In a small bowl, mix the cornflour with 2 tsp cold water, then stir into the gravy. Season, add 1 tsp butter and whisk together. Pour into a warm gravy jug.
To serve, brush the remaining sticky orange glaze over the duck, then put the duck on a board and garnish with the watercress. Heat a frying pan over a medium heat and add 1 tsp butter. Once the butter has melted, tip in the spinach and cook for 1-2 mins until it has wilted. Season and tip into a warm serving dish.