Put the potatoes in a pan of boiling water and cook for 5 minutes. Throw in the beans and cook for a further 3 minutes, by which time both should be just tender but not too soft. Drain and put to one side.
In a wok or large frying pan, heat the oil until very hot, then drop in the garlic and cook until golden, this should take only a few seconds. Don’t let it go very dark or it will spoil the taste. Spoon in the curry paste and stir it around for a few seconds to begin to cook the spices and release all the flavours. Next, pour in the coconut milk and let it come to a bubble.
Stir in the fish sauce and sugar, then the prawns. Turn the heat down to a simmer and cook, covered, for about 8 minutes until the chicken is cooked.
Tip in the potatoes and beans and pak choi and lemonrass and let them warm through in the hot coconut milk, then add a lovely citrussy flavour by stirring in the shredded lime leaves and lime zest. The basil leaves go in next, but only leave them briefly on the heat or they will quickly lose their brightness. Scatter with the lime garnish and serve immediately with boiled rice.