Cut the scallions into 2-inch lengths, and cut the pieces in half lengthwise. Julienne them thinly. Thinly slice about 15g of ginger, and julienne them. Give the cilantro a rough chop. Set the aromatics aside.
Combine the light soy sauce, salt, sugar and hot water in a small bowl and mix until the sugar and salt are dissolved. Set aside.
Prepare your steaming set-up, and fill with 1-2 inches of water. Bring to a boil.
Rinse your fish fillet, and carefully lay it on an oblong heat-proof plate that will fit into your wok or steaming setup. Carefully place it in the steamer, and adjust the heat to medium. The water should be at a slow boil that generates a good amount of steam, but not so high that the water evaporates too quickly.
Cover and steam for 7-10 minutes depending upon the size and thickness of your fish fillet. If you have extremely small, thin fillets (half an inch), cook for 4-5 minutes. Check for doneness using a butter knife. If it falls easily through the thickest part of the fillet to the bottom of the plate, the fish is done.
Turn off the heat, and carefully drain any liquid on the plate. Spread about ⅓ of the scallions, ginger, and cilantro on the steamed fish (alternatively, you can wait to do this AFTER adding the sauce).
To make the sauce, heat a wok or small saucepan to medium high heat, and add 2 tablespoons vegetable oil. Add the remaining ⅔ of the ginger, and fry for 1 minute. Add the white parts of the scallions and cook for 30 seconds.Then add rest of the scallions and cilantro. The mixture should be sizzling.
Add the soy sauce mixture. Bring the mixture to a bubble, and cook until the scallions and cilantro are just wilted, about 30 seconds.
Pour this mixture over the fish. If you prefer to add the raw aromatics after adding the sauce, you can do so now, and heat an additional 1 tablespoon of vegetable oil to pour over the raw aromatics. Serve immediately!