Mix all sauce ingredients in a saucepan, and bring to a gentle boil. Turn off heat, set aside.
Coat fish slices thoroughly in potato or corn starch. Shake off the excess and discard. Set aside.
Heat a wok with oil over high heat till hot. Fry the fish in two batches, constantly stirring and separating fish slices in hot oil. Deep fry till fish slices turn light brown and just begin to crisp at the edges. Sieve out into a wire basket or rack.
Wait for 10 to 15 seconds, and allow the oil to get very hot again. Put back the fried slices, and this time fry until golden brown. Repeat the process with the second batch.
Pour out oil, leaving 2 tbsp in the wok and heat up over medium fire. When hot, add diced onion and peppers, fry for about a minute. Add tomato slices and stir fry for another 15 to 20 seconds. Add the diced pineapple (or lychee halves) and stir fry for a few more seconds to mix well.
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. Bring the sauce to a boil.
Put in fried fish slices all at once, quickly toss a couple of times to coat fish evenly with sauce. Turn off heat, dish out onto serving plate. Serve immediately.
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