Blitz the onion, ginger, garlic and chilli in a blender or food processor to form a rough paste. Alternatively, chop the ingredients very finely by hand. Heat the oil in a saucepan, add the onion paste and fry for 2–3 minutes, stirring, until softened.
Stir in the curry paste and cook briefly, then mix in the coconut milk, fish stock and fish sauce. Simmer gently for 5 minutes. Allow to cool, then chill until required.
Soak the rice noodles in cold water until soft. Chill until required.
When ready to serve, reheat the coconut broth, add the fennel and green beans and simmer for 5 minutes. Tear half the coriander into pieces, add to the broth with the drained noodles, corn cobs and prawns and simmer for 4–5 minutes until piping hot. Ladle into bowls and garnish with the remaining coriander.