Place a large wok over a high heat. Once the wok starts to smoke, add the sunflower oil.
Add the ginger, garlic and chillies and stir-fry for 20-30 seconds.
Add the sweet potato and stir-fry briskly for 3-4 minutes.
Add the green beans and stir-fry for 4-5 minutes, until the sweet potato is tender and the beans are nicely browned.
Tip all of the vegetables out into a warm bowl and return the wok to the heat. When it is back up to prime heat, add the lamb and stir-fry for 1-2 minutes, until just barely cooked through.
Return the vegetables to the wok and mix them well with the lamb.
Add the black bean sauce and stir-fry for a final couple of minutes.
Add the sesame oil and stir well.
To serve, spoon into bowls.