Gently fry the onion, apple, garlic, ginger and chilli, if using, in a pan with the oil for about 10 mins until softened. Pour in the vinegar and bubble down until reduced by half. Add 100ml water and sugar to taste. Cook until reduced by two thirds and thickened. Stir in the redcurrants and balsamic vinegar, and cook for another 5 mins.
The relish can be made up to a week ahead and stored in the fridge.
Using a sharp knife, slash the fat of the duck breasts in a criss-cross, cutting almost through to the flesh. Rub the skin with salt and five-spice powder.
Heat a heavy-based frying pan, until you can feel a gentle heat, then place the duck, skin-side down. Cook for 5 mins on a low to medium heat until the fat begin to melt. Spoon over the duck and season.
When the skin begins to brown and crisp, after about another 2 mins, tip out the excess fat. Flip over the breasts and cook for another 5 mins on the flesh side. This will give you medium-rare duck. If you prefer well done, cook for a further 3 mins or so. Remove from the heat, allow to stand for 5 mins then slice diagonally. Serve with watercress salad and the relish.