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1
Beat the eggs thoroughly in a bowl. Warm the chicken stock to lukewarm — it must be neither cold nor hot, as either extreme will cause the custard to curdle. Pour the stock into the eggs in a steady stream, stirring gently, maintaining the 1:2 ratio of egg to stock. The mixture should be pale, even, and fully combined.
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2
Strain the mixture twice through a fine-mesh sieve into your serving bowl or bowls, removing all chalazae and air bubbles. The surface of the strained liquid should lie perfectly still and unblemished.
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3
Cover the bowl tightly with cling film or set a plate directly over it. This is where the dish lives or dies: the cover prevents condensation droplets from falling onto the surface and pocking it during steaming. Place in a steamer set over very gentle heat — ideally 60–70°C, never over vigorously boiling water. A folded towel laid over the steamer lid will absorb further condensation.
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4
Steam over this low, steady heat until the custard is just set. Insert a toothpick at the centre — it should come out just barely clean. The surface, when the cover is lifted, must be smooth as silk, with no pocking, bubbling, or cratering.
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5
Remove from the steamer. Finish by spooning oyster sauce and sesame oil over the surface, then scatter the sliced spring onion. Serve immediately in the vessel it was cooked in.