1. Wipe over the duck and remove giblets.
2. Rub salt and freshly ground black pepper inside the duck.
3. Melt half the butter in a flameproof casserole, and brown the duck on both sides.
4. Add the onions, salt and pepper to taste, cover and simmer over low heat for 1 hour, turning the duck often.
5. Cut turnip into pieces.
6. Melt the remaining butter in a frying pan and gently saute the turnips for 10 minutes, then add 3 1/2 fl oz (100 ml) of water.
7. Season with salt and cover the frying pan.
8. Simmer over low heat for 20 minutes, sprinkle in the sugar and cook for a further 10 minutes.
9. Allow the turnip pieces to caramelise on all sides by shaking the pan frequently.
10. When the duck is cooked, add the turnips to the casserole and simmer for 5 minutes.
11. Place the duck on serving dish and surround with the onions and turnips.
12. Pour over the cooking liquid and serve at once.