Bash the lemongrass flat with a rolling pin.
Peel the onion, then slice into thin strips. Cut the chicken into bite-size pieces or slice into strips.
Deseed the chilli, then chop.
Heat the oil in a pan and cook the star anise for 10 seconds over low heat. Add the onion and fry for 5 minutes, then add the garlic and stir for about 1 minute until soft.
Add the chicken pieces, lemon grass, chilli, fish sauce, sugar and lime juice, then pour in enough hot stock to cover the meat.
Bring to the boil, then, simmer for 20 minutes under cover, over low temperature. Remove the lid, turn up the heat and reduce reduced the sauce for 5 minutes.
Discard the star anise and lemon grass pieces. Season to taste.
Serve hot with steamed rice and some chopped fresh coriander leaves scattered on top.