Source: HowToCook (a programmer's guide)
The point of onsen tamago is precise temperature-control: the whites set at 65°C whilst the yolk stays custard-soft at 70°C. This is gentle-heating, and it demands a thermometer you trust and water you don't abandon.
Fill a pot with enough water to submerge the egg by 3 cm. Clip a probe thermometer to the side — the reading must be at the centre of the water mass, not touching the pot base or sides. Bring the water to 70°C over medium heat, stirring occasionally to distribute warmth evenly. This takes ten to fifteen minutes depending on your pot and hob. Once the water holds steady at 70°C for two minutes, lower the egg gently into the water using a spoon. The water temperature will dip slightly; maintain the heat to bring it back to 70°C within a minute, then hold it there.
The egg needs exactly 25 minutes at temperature. The whites coagulate from the outside in — you'll see nothing from the outside, but the protein matrix is tightening. The yolk barely sets; it stays fluid or barely tremulous depending on the exact heat distribution inside the pot. Don't stack eggs or crowd the pot; they need space for the water to circulate and cook them evenly. If you're cooking more than one, you'll need to extend the time slightly — add two minutes per extra egg.
Prepare a bowl of ice water whilst the egg cooks. At 25 minutes, transfer the egg immediately to the ice bath with a slotted spoon. Leave it there for three minutes to arrest the carryover cooking and stop the whites from setting the yolk any further. The ice shock is non-negotiable; skip it and you'll end up with a hard yolk.
Crack the egg into a small bowl or over rice. The white should be barely set and slippery, the yolk liquid. Finish with a pinch of fleur de sel, a few drops of soy sauce or mirin, and a scatter of nori or chives if you're building a rice bowl. Eat it immediately — this dish doesn't improve with time.
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