Heat the curry paste in a dry saucepan over a medium heat just for a few moments until the aroma begins to fill the room. You will need to stir constantly because there is no oil, or it will catch. Then pour in coconut water and chicken stock.
Remove the greens part of the spring onions and chop then set aside. Split the white part in half lengthways and add to the soup, along with the fish sauce and soy sauce and allow to come to boil when it reaches boiling point add the noodles. |cook noodles in the broth until tender. The starches from the noodles will very slightly thicken the soup. Squeeze in juice of ½ lime.
Using tongs or pasta claw remove noodles and split between two bowls. Pour broth over and sprinkle with chilli flakes, green tops of the spring onions and coriander. Cut the remaining ½ lime in half and serve alongside for squeezing over as you eat.