First make the salsa: heat the oil in a small frying pan, add the onions and garlic and cook over a high heat for 1-2 minutes until lightly browned. Remove and leave to cool.
Mix together the chillies, plums, pepper and coriander with the lime juice, add salt and pepper to taste, then stir in the onions and garlic. Set aside for at least 30 minutes, or overnight, to allow the flavours to develop.
Cut the skin and fat from the duck breasts and slash them several times.
Lightly brush a ridged, iron grill pan or heavy nonstick frying pan with oil and heat it until you feel a steady heat rising. Fry the breasts for at least 3 minutes on each side, or until they feel springy when pressed with the back of a fork. Larger breasts will need longer cooking, and the length of time depends on how pink you like to serve your meat.
Sprinkle the breasts lightly with salt, pepper and 5-spice powder, then leave them to stand for 5 minutes.
Spoon the salsa in mounds onto four dinner plates. Slice the duck breasts diagonally and arrange them on top of the salsa. Garnish with sprigs of coriander.