Pour the water or stock into a pan and add the lemon grass, torn lime leaves, chilli, ginger, fish sauce and sugar.
Bring to the boil, then reduce the heat to very low and leave to infuse for 10 minutes.
Cut the white fish into 1cm square cubes and drop into the broth.
Quickly heat a griddle or frying pan and rub the scallops with a little oil.
Carefully place scallops in the pan, and don't touch for three minutes, until you can see the scallops colouring around the edges.
Turn them over and cook for a further three minutes.
Distribute the coriander and basil among six little bowls. Squeeze a little lime juice over the top and pour in the soup.
Place a scallop in each bowl and garnish with a stick of lemongrass.