Heat your oven to 190°C/Fan 170°C/Gas 5. Pat the duck legs dry with kitchen paper. Prick them all over with a skewer or fork. Rub a little salt and pepper into the skin. Lay the duck legs, skin-side up, on a wire rack in a roasting tin. Roast for 1 hr till golden and crisp.
Meanwhile, peel and finely chop the onion. Peel and halve the shallots. Trim, peel and dice the carrot. Trim and finely slice the celery stick. Peel the turnip(s) and potato and dice them into roughly 1 cm large pieces.
Warm ½ tbsp olive oil in a deep frying pan or wok. Add the onion, carrot, shallots, celery and turnip(s). Season with salt and pepper.
Cook over a medium-low heat, stirring now and then, for 10 mins till the veg have softened but not picked up too much colour. If it starts to brown, turn the heat down. If it starts to stick, add a splash of water.
While the veg cook, peel and finely chop the garlic. Finely chop the parsley stalks and leaves, but keep them separate.
Stir the garlic and parsley stalks into the veg with the potato. Crumble in the stock cube. Pour in 250ml boiling water. Cover and simmer for 15 mins till the veg are tender.
Drain the beans and add them to the veg. Cover and heat for 5 mins to warm the beans through. Stir in most of the chopped parsley leaves. Taste and add more salt and pepper if you think it needs it.
The duck legs should be crisp and golden. Ladle the bean cassoulet into warm shallow bowls and top with a duck leg. Scatter over the remaining parsley. Any duck fat left in the roasting tin can be scooped into a tub and kept in the fridge for roasting potatoes.