Put the stock, lime leaves, ginger, chilli and lemon grass in a large saucepan. Cover and bring to the boil. Add the mushrooms, reduce the heat and simmer for 2 minutes.
Break the noodles into short lengths, drop into the soup and simmer for 3 minutes. Add the spinach and prawns and simmer for 2 minutes until the prawns are heated through. Add the lemon juice. Remove and discard the lemon grass stalk and season with pepper before serving.