To make the Thai green curry paste, put the lemon grass and lime juice into a small bowl and leave to soak for 30 mins. Put a small pan over a medium heat and add the peppercorns, coriander and cumin seeds. Dry fry for about 1 minute until fragrant then tip onto a saucer to cool.
Put the lemongrass, spices and all remaining curry paste ingredients into a mini food blender or small food processor and pulse the mixture. Scrape down the sides from time to time. Process as smooth as you can – the mixture will be slightly coarse and not as green as the purchased variety. Put the paste into a lidded jar and store in the fridge up to a week or freeze in portions.
To make the curry, open the cans of coconut milk and carefully remove the thick cream and spoon it into a warm wok or large frying pan (see tip below). Cook it slowly, stirring all the time, until it reduces to a thick paste and the oil starts to separate. Add the green curry paste and cook for 1-2 mins. Add the chicken and stir until well coated in the paste.
Slowly add the coconut water remaining in the cans together with the chicken stock. Add all the remaining ingredients and cook over a medium heat for about 10 mins until the chicken is cooked. Add more stock if necessary – the sauce should be slightly thinner than single cream.
To serve spoon some rice into a bowl, pour over the curry and sprinkle with the coriander leaves, sliced chilli and spring onion. Add a wedge of lime on the side.