1. Chop potatoes into even bite-sized chunks, place in a medium saucepan of boiling water and cook for 5 minutes, or till softened, then drain.
2. Heat the oil in a large frying pan over medium heat and cook the onion for about a minute. Add the curry paste and saute for 2 minutes to release the aromas.
3. Pour in the coconut milk and some water and let it come to a simmer for a further 5 minutes.
4. Stir in the fish sauce and sugar, then add the chicken. Turn down the heat slightly, cover and simmer for about 8 minutes or until the chicken is cooked through.
5. Add the potatoes, green beans, lime zest and juice and finally mix the cornflour with 1 tbsp of the sauce from the dish then stir. Simmer for a final 5-8 minutes till thickened slightly and heated through.
6. Garnish with freshly chopped coriander or basil, chilli flakes and peanuts/cashews if using.