Peel and finely chop the onion, garlic and ginger. Halve and deseed the chilli, then cut into slivers.
Heat the oil in a large saucepan or wok. Add the onion, garlic, ginger and chilli. Stir-fry over medium heat until the onion is soft. Add the coconut milk, lime juice, sugar and kaffir leaf. Bring to the boil.
Meanwhile cut each green bean into 3 bits, first removing the ends. Add these to the curry and cook for 5 minutes. Add the prawns, put a lid on, turn up the heat and cook for about 3- 5 minutes, then turn off the heat, stir in the coriander. Put the lid on back quickly and rest for a couple of minutes.
Serve with the chilli and coriander leaves scattered on top.