Heat oil in a large saucepan on low. Cook ginger for 2 minutes. Add 1.5 litres water, fish sauce, brown sugar and soy sauce. Increase heat to medium and simmer for 5 minutes.
Add salmon and poach gently for 2-3 minutes, until fish is just cooked. Remove from broth and place in serving bowls. Add green onion, pak choy and lime juice to broth and simmer for 1 minute, until pak choy is tender.
Ladle ginger broth and a little pak choy over salmon. Garnish with coriander leaves and chilli.