A soy-based sauce for steamed fish
Here, Cantonese fish sauce means the ginger-scallion and soy finish served with steamed fish. It is not a recipe for fermented bottled fish sauce. Do not substitute a large quantity of fermented fish sauce for the light soy below: those are different condiments.

Sauce for a whole fish of about 680 g
Made With Lau’s sauce combines 2 tbsp light soy sauce, half a tsp dark soy, half a tbsp cooking wine, a quarter tsp white pepper, 1 tbsp water, half a tsp sugar and half a tsp salt. Its finish also uses 2 tbsp neutral oil and 1 tbsp sesame oil. The fish is garnished with 28 g ginger, five scallions and five cilantro sprigs. Adjust added salt for your soy brand rather than assuming all brands have the same strength.
Separate steaming aromatics from the finish
Use a cleaned, scaled and gutted fish that fits its heatproof plate and steamer. Slice half the ginger for steaming; cut the remaining half into fine strips for finishing. Put scallion greens under the fish and ginger slices over it. Mix the sauce separately so raw-fish handling does not contaminate the serving bowl.

Cook the fish before adding the finishing sauce
Start over fully boiling water. The source checks a fish this size after about 12 minutes of covered high steaming, but thickness changes timing. Check the thickest flesh away from bone reaches 63 C. Carefully remove the plate, discard the steaming liquid and spent ginger, and add fresh ginger and scallion strips.
Heat the neutral oil until shimmering, not smoking. Carefully pour it over the aromatics while the plate is stable and your hands are out of the splash path. Add the prepared sauce, sesame oil and cilantro. The oil and sauce finish is not a substitute for cooking the fish.

Frequently Asked Questions
Is bottled fish sauce the same as seasoned soy for fish?
No. Read the label. A product labelled seasoned soy sauce for steaming fish is soy-based; fermented fish sauce is a separate ingredient with a different flavor and concentration.
Can I use a fillet?
Yes, but scale the sauce to the amount of fish and shorten the first cooking checkpoint. Keep the 63 C doneness check rather than using the whole-fish timer unchanged.
Menu and sources
Check fish, soy, wheat and sesame allergens. Pair with claypot rice or a separate steamed chicken plate.
Recipe and condiment distinction: the Lau family’s steamed fish. Safety: official fish temperatures.