1Cook the noodles following pack instructions. Refresh in cold water and drain well.
3Put the stock in a large saucepan and stir the lime leaves, fish sauce and 250ml cold water. Bring to a simmer and cook for 5 mins.
5Cut the fish into roughly 3cm cubes and add to the pan. Return to a simmer, then cook for 2 mins uncovered.
7Stir in the noodles, prawns and pak choi, and simmer for 2-3 mins or until the fish and prawns are just cooked.