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1
Bring a lidded pot of water to a full boil, then remove from heat. Verify the temperature with a thermometer: the target is 65–68°C. In a warm kitchen, a lidded pot of just-boiled water will hold approximately 70°C for 30 minutes or more without a heat source; adjust accordingly, or use a sous-vide circulator or rice cooker keep-warm setting verified by thermometer to hold steadily at 65–68°C. A deviation of even 3–4°C above 72°C will produce rubbery whites and forfeit the dish's defining quality.
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2
Lower the cold eggs — straight from the refrigerator — gently into the water using a slotted spoon. The cold core creates the internal temperature gradient that assists the textural inversion. Cover the pot and hold at 65–68°C for a minimum of 45 minutes. Below 25 minutes, whites will remain raw; 45 minutes is the most reliable duration across egg sizes and kitchen variables. This is where the dish lives or dies: sustained precision temperature over sufficient time produces the custardy, barely-set white and jammy, flowing yolk that define onsen tamago — no other step can compensate for a failure here.
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3
While the eggs cook, combine the dashi, soy sauce, mirin, and sake (if using) in a small saucepan. Warm gently over low heat until just steaming; do not boil. The kakejiru should smell clean and savory — dashi-forward, with soy providing seasoning rather than dominance. Remove from heat and hold warm.
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4
Remove the eggs from the water and serve immediately, shell-on, in small individual bowls. At the table, crack each egg into its bowl and pour the warm kakejiru tsuyu over. The white will appear silky, barely cohesive, and almost translucent at the edges; the yolk will be deeply colored, jammy, and structurally firmer than the white — the textural inversion that is the egg's signature. A white that is fully set and opaque indicates the water exceeded 72°C; a white that is entirely liquid indicates insufficient time or temperature.