Make up 1½ litres of vegetable stock in a large deep saucepan and add the sliced fresh ginger, 1 halved red chilli, the spring onions and the soy sauce. Bring to a boil, then turn down the heat and leave to simmer for 15 minutes.
Remove the ginger and chilli from the broth and discard, then add the pak choi to the pan and cook for 2 minutes.
Cook the noodles according to instructions on the packet. Divide between two serving bowls, then ladle over the broth and pak choi. Add the cooked leftover salmon, then garnish with the fresh mint and basil leaves and the remaining red chilli, finely sliced.