Heat half the oil in a wok over a high heat until the oil starts to shimmer. Add the ginger, garlic and chillies and stir-fry for a few seconds before adding the sweet potato. Stir-fry for 3 minutes, add the French beans and stir-fry for a further 3–4 minutes, until the sweet potato is tender and the beans are golden. Tip the vegetables on to a large plate and set aside while you cook the lamb.
Return the wok to the heat, wipe it clean with some kitchen paper and heat the remaining groundnut oil. Add the lamb and stir-fry for 2 minutes, then stir in the oyster sauce and measurement water. Cook, stirring, for 1 minute, then toss in the cooked vegetables and stir-fry for a final minute. Pour in the sesame oil, stir well to combine and serve.