Heat the oven to gas 7, 220°C, fan 200°C. Place the duck breasts on a board and score the skin in a criss-cross pattern with a sharp knife, then season well. Heat a griddle pan or frying pan until hot, place the duck breasts in it, skin-side down, and cook for 5 minutes.
Crush the coriander, fennel, star anise and a pinch of chilli flakes in a pestle and mortar. Rub the skin-side of the duck breasts with the spice mix, then place in the roasting tin along with the plums. Mix the soy, honey and sugar, and pour over the duck breasts.
Place in the oven and bake for 10 minutes, until the duck is tender but still a little pink in the centre.
While the duck is cooking, pierce the sweet potatoes with a knife, then microwave on full power for about 7 minutes, until tender. Cut in half, scoop out the flesh and mash with the butter, a pinch of chilli flakes and a generous grinding of black pepper.
To serve, slice each duck breast into four and serve with the halved plums and sweet-potato mash.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.