Heat the oil in a wok or large frying pan. Add the green curry paste and sugar and cook over a fairly high heat for about a minute, stirring with the lemongrass, if using. Reduce the heat slightly and stir in the chicken pieces and lime leaves or zest until coated in the paste. Add the coconut milk, fish sauce or soy sauce and bring to a simmer, cooking for 25-30 minutes until thickened slightly. Stir in the coriander and lime juice. Check for seasoning, adding more fish sauce or soy sauce if needed.
The curry is now best left to sit for a few minutes so the sauce becomes creamier. You will also taste the true flavours of the curry paste ingredients when it's slightly cooler. Serve with lots of fragrant Thai jasmine rice.