Cook the rice according to the packet instructions. Drain and leave to cool, then chill until needed.Pick half of the coriander leaves, pop them in a bowl of ice-cold water and set aside. Put the remaining leaves in a food processor along with the cooled rice.Thinly slice the spring onions. Roughly chop the chicken, bash and roughly chop the lemongrass, then peel and roughly chop the ginger. Peel the garlic.Place it all into the food processor with the lime leaves (remove the stems) and pulse until smooth.Tip the mixture onto a board. With wet hands, divide it into 16 pieces and roll each into a ball. Place them on a plate and chill, covered, until needed.Place a large casserole pan over a medium-high heat and add a splash of sunflower oil. Fry the rice balls for 5 minutes, or until golden brown all over.Peel the prawns, using a sharp knife to remove any black bits, then add to the pan and stir-fry for 1 minute. Stir in the miso paste and pour in 1.2 litres of boiling water. Simmer for 10 minutes.Cut each pak choi into 6 pieces, then halve the mangetout lengthways. Add them to the pan for the last 2 minutes. Stir in the beansprouts for the last 30 seconds and season with a splash of soy sauce.Thinly slice the remaining spring onions and finely slice the chilli. Divide the broth, balls, veg and prawns between four warmed bowls. Sprinkle over the spring onions, chilli and drained coriander and serve immediately with lime wedges.