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1
Bring the steamer to a bare simmer — not a rolling boil — so the gentle, even heat can set the custard smoothly rather than seizing it into curd. Test the level by holding a hand briefly above the steam: if it is just tolerable, the temperature is right. Hold at this gentle level throughout.
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2
Break the eggs into a bowl and beat them slowly and deliberately with chopsticks or a fork, working in long strokes rather than a whisk, so that air stays out of the mixture and the surface of the finished custard remains unmarked by pits. The goal is a fully combined mixture with no foam visible on the surface.
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3
Add the warm water or dashi at 40 °C to the eggs in the ratio of 1.5 volumes liquid to 1 volume egg — a higher proportion yields a more delicate, barely-there texture — and stir gently to combine. Pass the mixture through a fine sieve into the serving bowl or bowls, pressing out any chalazae, so that no threads or foam survive to mark the surface of the set custard. The strained liquid should be entirely smooth.
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4
This is where the dish lives or dies. Lower the bowl into the steamer and lay a clean cloth or a sheet of foil loosely over the bowl — not the steamer lid — so that condensation drips onto the cloth rather than pitting the custard's surface. Steam over the barest simmer, on no account allowing the heat to climb to a rolling boil, which would scramble the eggs into curd rather than setting them to custard; continue until the edges are just set and the centre still trembles when the bowl is nudged.
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5
Remove the bowl from the steamer. Dress the surface with a few drops of light soy sauce and sesame oil, then scatter the sliced spring onion. Serve immediately in the bowl.