Heat the oven to 200°C/Fan 180°C/Gas 6. Rinse and peel the potatoes and parsnips. Chop them into 1/2 cm thick slices. Peel and grate or crush the garlic. Strip the thyme leaves from half the stems.
Fill and boil the kettle. Pop the potatoes, parsnips, garlic and thyme in a roasting tin. Season well. Gently toss everything together to mix the flavours.
Pour 200ml hot water from the kettle over the sliced veg. Use a fork to prick the skin of the duck legs. Rub them with a pinch of salt and pepper. Pop them on top of the veg.
Loosely cover the roasting tin with foil. Roast for 20 mins. While the duck and veg cook, make the apple sauce. Quarter, core and peel the apple. Roughly chop it. Pop in a small pan with the sugar.
Finely grate or pare the zest from the orange. Juice it. Add the zest and juice to the apple. Cover. Bring slowly to the boil. Simmer for 3-5 mins till the apples are soft and pulpy. Stir with a spoon to break the sauce up.
Take the foil off the roasting tin. Pop the duck back in the oven. Roast for 30-40 mins till the duck legs are golden and crisp.
When the duck is nearly cooked, rinse the cavolo nero. Trim off the tough ends of the stalks. Thickly shred the leaves. Pop them in a pan. Cover with hot water from the kettle. Boil for 4-5 mins till cooked but not too soft.
Drain the cavolo nero. Spoon the sauce into a serving bowl. Divide the potatoes and parsnips between two plates. Top with the duck and serve with the cavolo nero and orange and apple sauce.