Stir the soy sauce, hot water, and sugar together until the sugar dissolves. This is the seasoned soy that finishes the dish.
Lay the ginger coins and scallion whites on a heatproof plate that fits inside your steamer or wok. Set the fillets on top so steam can circulate underneath them.
Steam over rapidly boiling water for 7 to 9 minutes depending on thickness. The fish is done when it flakes at the thickest point. Start checking at 7.
Carefully pour off all the liquid that collected on the plate and discard the spent ginger and scallion from underneath. That liquid is where the fishy taste lives.
Pile the fresh ginger and scallion threads on top of the fish.
Heat the oil in a small pan until it shimmers and just begins to smoke, then pour it directly over the threads. They should sizzle loudly. Pour the seasoned soy around the fish, not over it, and serve with rice.