After cleaning the fish, trim the tail and fins, and make slits on the fish.
Combine the water, spring onion stems, young ginger, and salt. Rub it all over the fish.
Stuff the spring onion stem and ginger in the stomach of the fish and marinate for 10 minutes. After that, discard the spring onion stem and ginger.
Dust corn flour on one side of the fish and let it rest for 2-3 minutes. Flip to the other side, and dust corn flour.
Heat up the cooking oil to shallow fry the fish.
Once the oil is hot, carefully lower the fish into the wok and fry for 3 minutes per side over medium heat until light golden brown on both sides. Then place the fish on a serving plate.
Remove most of the oil from the wok and leave about 2 tablespoons.
Add in ginger and garlic, stir-fry until fragrant over low heat.
Add tomato sauce, sugar, salt and water, then turn the heat up to high and bring the sauce to a boil.
Once the sauce is boiling, add tomatoes, pineapple and cucumber, and cook for two minutes.
Add the vinegar and thicken up the sauce with some cornstarch solution.
Pour the sauce over the fried fish and serve while it is hot.