KO EN Japanese Side dish⏱ 55 minEdit history

Onsen Tamago (Slow-Set Egg)

Name: Onsen Tamago (Slow-Set Egg)
Prep time: 10 min
Cook time: 45 min
Total time: 55 min
Yield: 4 servings
Cuisine: Japanese
Ingredients: 4 eggs; 1 L water; 200 ml cold water; 100 ml dashi; 1 tbsp soy sauce; 1 tbsp mirin; 1 spring onion; 1 pinch shichimi

The Japanese egg with a set yolk inside a white that still runs — the exact reverse of a boiled egg, and the only reason is that there is no temperature gradient. A boiled egg sets its white first because the outside is at 100°C while the middle is still cold, not because the white is the quicker protein. The bulk of the white needs close to 80°C to firm up and the yolk is done by about 70, so hold the whole egg at 68-70°C and the order reverses. What this recipe has is not a timing but a temperature to hold.

Ingredients

4servings

Steps

Bring the dashi, soy and mirin to the boil once to make the tsuyu, then let it cool. Poured over hot, it cooks the loose white on contact.
⏲ 10 min
Take the eggs out of the fridge and leave them at room temperature for 30 minutes. Start from 4°C and the water cools past the window before the centre ever reaches it.
⏲ 30 min
Boil 1L of water in a heavy pot with a lid. A thin pan will not do this — holding the heat is the only thermostat in the recipe.
⏲ 10 min
Off the heat, pour in 200ml of cold water to bring it to 68-70°C. Measure it if you have a thermometer; roughly five parts boiling to one part cold lands there.
⏲ 5 min
Lower the eggs in, put the lid on and leave them off the heat for 25-30 minutes. Do not put the pot back on the heat. Above 80°C the white sets and you have an ordinary soft-boiled egg.
⏲ 30 min
Lift them out and hold them in cold water for a minute to stop the carryover.
⏲ 2 min
Crack one into a small bowl. The white should run out loose and the yolk should hold its shape — that is the whole test. Pour the cooled tsuyu around it and add the spring onion and shichimi.
⏲ 3 min

Tips & Variations

Variations

On rice and noodles: one dropped onto gyudon, curry rice, udon or bukkake udon is what it is most often for. The loose white becomes the sauce.
Longer and cooler: an hour at 63°C gives a white that clings a little more and a yolk that is closer to a custard. With a sous-vide circulator this is the more repeatable version.
At the source: where a hot spring runs at 65-75°C the eggs are simply lowered into it in a basket, which is where the name comes from and what is sold at Yunomine and Beppu.
Not the black eggs: the kurotamago of Owakudani are hard-boiled eggs whose shells have gone black from sulphur compounds. Neighbours in name only.
The opposite of tea eggs: those keep the shell on so that the liquid can reach only where it is cracked. Same egg; the difference is whether the shell is used as a mask.

Tips

Having no temperature gradient is the entire dish. Cooking eggs gives the two numbers: white begins to set at about 62°C, yolk at about 68°C. Those are the starts. The order in which they finish is the other way round — the protein that makes up most of the white needs close to 80°C to coagulate properly, while the yolk is done by about 70. So held at 68-70°C, only the yolk sets.
A boiled egg comes out the other way for a different reason. The 6, 7 and 9 minute table in that same guide is measuring a gradient: the shell is at 100°C while the middle is still cold. Remove the gradient and the sequence inverts.
The pot is the thermostat. A heavy pan with the lid on drops only a few degrees over 25 minutes. A thin one, or one left uncovered, is under 60°C within fifteen and the yolk never sets either. A vacuum flask works too.
Do not put it back on the heat. The moment you light the burner because it feels cool, the water at the base goes past 80°C and the white in contact with it sets. If it has fallen too far, add a splash more boiling water instead.
Against scotch egg. That one hits two centres a centimetre apart by staggering them in time — the yolk is set to 6:30 first, chilled hard, and then fried. This one has a single centre and instead removes the gradient. Two answers to the same pair of numbers.
Very fresh eggs peel badly — irrelevant here, because nothing is peeled; the egg is cracked straight into the bowl. The only advantage a fresh egg has in this dish is a white that spreads less.
Judge it by shape, not by the clock. Crack one: a white that pours like a thin soup and a yolk that holds together means it is right. A white gone opaque and firm means the water was too hot.