Cut the chicken into bite-sized pieces and fry until browned.
Remove the hard outer leaves of the lemongrass and discard. Chop lemongrass. Add together with the garlic, curry paste and lime zest and fry another couple of minutes.
Pour in the coconut milk, chicken stock and lime juice (and fish sauce, if using). Turn down the heat, cover with a lid and simmer for 10 minutes.
Add the sugarsnap peas and cook another 2 minutes. Then add the beansprouts and cook a further 1 minute (no longer, otherwise they become soggy).
Sprinkle with coriander leaves before serving. Great with cauliflower rice.