Heat the groundnut oil in a large, heavy-based saucepan set over a moderate heat. Saute the spring onions and shallot for 2-3 minutes, then add the garlic, ginger, red chilli and lemongrass. Continue to cook over a reduced heat, stirring occasionally, for 2-3 minutes.
Pour in the stock and lime juice and bring to a simmer, then add the chunks of cod and simmer for 3-4 minutes. Stir in the soy sauce, caster sugar and fish sauce and cook for a further 1-2 minutes to make sure the sugar has dissolved. Remove from the heat and stir in the chopped coriander.
Discard the lemongrass, then ladle into soup bowls. Garnish with black pepper and sprigs of coriander before serving.