Mix all sauce ingredients in a saucepan, and bring to a gentle boil. Turn off heat, set aside.
Clean and pat dry fish fillet paper towels. Slice into 5-mm thick slices. Coat fish slices thoroughly in corn flour, shake off the excess flour and discard. Set aside.
Peel and dice onion. Seed and dice peppers, tomato and pineapple.
Fill a wok to 1/4 depth with oil and heat up till hot. Fry the fish in two batches, constantly stirring and separating fish slices in hot oil with a wok chan or stainless steel ladle. Deep fry till fish slices turn golden brown and just begin to crisp at the edges. Remove, drain onto paper towels. Let oil heat up again, before frying the second batch.
Pour out oil, leaving 2 to 3 tbsp in the wok and heat up over medium fire. When hot, add diced onion and peppers, fry for about a minute. Add diced pineapple and tomato, stir fry for another 15 to 20 seconds, then add the sauce. If the sauce is too thick, add 2 to 3 tbsp water (the sauce should still be slightly thick and sticky, not watery), stir quickly to mix well. Put in fried fish slices all at once, quickly toss for another 20 seconds to coat fish all over with sauce. Turn off heat, dish out onto serving plate. Serve immediately.