Season the fish fillets evenly on both sides with salt. Cut the fish into bite-sized pieces and lightly dust with the cornflour to coat evenly.
Spray a nonstick wok or large frying pan with cooking spray and heat over a medium heat. Add the fish and fry for 1–2 minutes on each side until lightly browned. Remove from the pan to a plate.
Wipe the pan clean with kitchen paper, re-spray with cooking spray and heat over a high heat. Add the spring onions, garlic and ginger and stir-fry for 30 seconds.
Stir all the sauce ingredients into the pan and season to taste with salt and white pepper. Bring the mixture to the boil, then reduce the heat to a simmer and return the fish to the pan. Simmer for about 4–5 minutes until the fish is cooked through, then serve immediately with steamed rice.