Heat the oil in a frying pan over a medium heat and fry the duck breasts, skin-side down, for about 15 minutes until the skin is really crispy. Drain off the excess fat, turn the duck over and fry for a further 5 minutes. Remove and keep warm.
Put the spring greens in a steamer over a pan of boiling water and steam for 2–3 minutes or until wilted.
Add the remaining ingredients to the frying pan and season with salt to taste, then stir to mix and allow to bubble for 2–3 minutes.
Serve the duck breasts with the sauce poured over and with the steamed spring greens.