Preheat the oven to 190 C, 170 Fan, Gas 5. Line a baking tray with some baking paper.
Drain the potatoes and pat very dry. Coat in the butter or duck fat, sprinkle with the thyme and season with salt and pepper.
Make a circle of the potato slices on the tray, then a slightly smaller circular layer in a different rotation. Build up a further 4 layers, each slightly smaller and on a slightly different rotation so it looks a bit like the petals of a flower.
Bake for 30-35 minutes until the potatoes are golden brown at the edges and tender.
Meanwhile, using a sharp knife score the fat of the duck breasts in a criss cross pattern all over and season well with salt and pepper. Place the breasts skin side down in an ovensafe frying pan.
Place over a moderate heat and cook for about 8 minutes, until most of the fat has rendered down and the skin is crisp and brown all over.
Turn over the duck breasts and return the pan to the oven. Cook for 5-6 minutes or until rare in the middle. Transfer the duck breasts to a plate and rub all over with the chopped stem ginger. Set aside to rest somewhere warm while you make the sauce.
Pour off all but 2 tbsps duck fat from the frying and set over a moderate heat. Add the shallot and cook for about 2 minutes, stirring briskly, then add the honey, blackcurrant jam and finally the vinegar.
Cook for about 3 minutes, stirring from time to time, until reduced to a syrupy sauce. Season with salt and pepper.
Return the duck breasts to the pan, skin sides down, reducing the heat to medium low. Spoon or brush the sauce over the breasts to coat evenly.
Turn skin side upwards and spoon the sauce over for about 30 seconds. Remove from the heat and let the duck breasts rest for a final five minutes in the pan before slicing.
While the duck is resting dress the watercress leaves with walnut oil and some red wine vinegar to taste. Sprinkle with flaky salt.
Serve the duck with the sauce from the pan, the pommes anna and the dressed watercress.