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1
Prepare the water bath. Fill a vessel deep enough to submerge the eggs with water and bring it to a steady 65–68°C. A rice cooker on its 'keep warm' setting or a sous vide circulator set to 64°C achieves this reliably. Verify the temperature with a thermometer before proceeding; fluctuation above 70°C will produce a conventional soft-boil, below 63°C the white will remain raw. The bath is ready when the temperature holds without deviation.
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2
Lower the eggs, still in their shells, gently into the water bath. This is where the dish lives or dies: maintain 65–68°C without interruption for the full 20–25 minutes. At this temperature, the yolk proteins — which coagulate at approximately 62°C — set to a smooth, creamy consistency, while the white proteins — which require 70°C or above to firm — remain custardy and barely cohesive. A steady 65°C for 25 minutes produces the most consistent result. Do not cover and walk away without monitoring; temperature drift decides everything.
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3
While the eggs cook, combine the mentsuyu concentrate and dashi in a ratio of 1 part mentsuyu to 3 parts dashi. Stir briefly and hold warm. The tsuyu is ready when it smells distinctly savoury — round with bonito and kombu — and tastes of balanced salt and umami without sharpness.
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4
Remove the eggs from the bath at the close of the cooking window. Crack each egg carefully into a small, room-temperature bowl — not a warmed one, as residual heat will continue to cook the egg. The white should tremble and barely hold its form around a fully set, creamy-smooth yolk. If the white is fully liquid, the time was insufficient; if the yolk runs, the temperature exceeded the window.
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5
Pour a small measure of the warm dashi tsuyu around the egg in the bowl and serve immediately. Onsen tamago deteriorates on standing; refrigeration sets the white firm and the texture is irreversible. The dish is complete when the egg's ivory white trembles at the edge of liquid and the yolk sits smooth and intensely eggy-creamy at its centre.