Rub both oils, 5-spice powder, salt and pepper over the duck breasts. Put the breasts skin side down in a cold wok, then bring it slowly up to a medium-low temperature so the white fat turns into wonderful thin, crispy, golden crackling. Cook for 10–12 minutes, then turn the breasts over and cook for a further 5 minutes. Rest for 5 minutes before slicing.
Peel and finely shred the mangoes or apple just before mixing (to prevent their colour darkening) and mix with the duck slices, cucumber, tomatoes, shallots, ginger, soy sauce, lime juice, chillies and cashew nuts. Taste and adjust the seasoning.
Pile mixed salad leaves on to each serving plate. Spoon the duck and cashew nuts on top and serve as a salad, side dish or starter.