Top and tail the beans, string and cut into 2.5cm lengths if necessary, depending on the size and age of the beans. Blanch in boiling salted water for two minutes.
Drain the beans and immediately plunge them into cold water to prevent further cooking. When cold, drain well and pat dry with kitchen paper.
Pour two tablespoons of oil into a non-stick frying pan or wok and place on a high heat. As soon as it starts to smoke, add the asafoetida, mustard seeds, cumin seeds and curry leaves. As soon as they begin to pop and splutter add the onions and garlic.
Cook til the onions become soft and begin to brown at the edges. Now add the beans and heat through continuing to stir.
Finally add the salt, sugar, lime juice and toasted sesame seeds. Toss everything together and adjust seasoning if needs be and serve immediately.