After coming up with, what I thought were reasonably safe conditions for pasteurizing shell eggs at home using Sous vide, the next natural step was to make Japanese "Onsen" eggs 温泉卵. One thing I have to point out is that a Japanese "onsen" egg is not a Western-style soft boiled or poached egg. For Westerners, a totally cooked but still tender white with a runny egg yolk are the combination that constitutes a perfect soft "boiled" or "poached" egg. Japanese onsen eggs are just the opposite—the white is still loose and a bit slippery but the yolk is congealed to the texture of thickened cream or custard or "nettori" ねっとりtexture and moist. To make this combination you exploit the fact that the yolk congeals at a slightly lower temperature than the white. Also another fact to consider is that the "jelling" process of egg proteins occur slowly and progressively and the duration of cooking also make a difference in the end results.
The above is my sous vide "onsen" eggs. I served it cold with cubes of ripe avocado, garnished with finely chopped scallion, and "real" wasabi. I poured cold sauce which I made over it. The green is water cress added for color.
The above is how the yolk looks after cutting into it using a spoon. It has a viscous and moist texture but is not runny. The white is not firmly congealed and is very tender still slightly slippery but does not have a "raw" texture. As far as I am concerned this is a perfect "Onsen" egg. You could make this without Sous vide but with it, you can control the temperature precisely to make perfect and safe "Onsen" egg every time.
I consulted the blog by a Japanese molecular scientist and gastronome who described making the "ultimate onsen egg " 究極の温泉卵 using a circulator/water bath with 12 combinations of temperature and duration (in Japanese). I followed his recommendation of 67C (152.6C) for 30 minutes. Again there is no mention of the initial temperature of the eggs in this post. I used eggs directly from the refrigerator (40F or 4.4C) and after 30 minutes were up, immediately submerged the cooked eggs in ice cold water for for 15 minutes, wiped them dry and stored them in the refrigerator.
With this high-temperature of 67C in sous vide, I estimated that the egg yolk will reach 54.4C sometime conformably before 30 minutes, extrapolating from the Dr. Cox's chart at 60C. The fact, also, that the egg yolk was homogeneously congealed indicates to me the temperature of the egg yolk was way above 54.4C*, probably near or at 67C, without significant temperature gradients. Although I used Pasteurized shell eggs this time, I am reasonably comfortable that I can safely used regular shell eggs for this recipe.
*Requires 5 minutes at 54.4C to accomplish "lethal" kill of salmonella, reportedly.
Sauce: I had leftover dashi broth which I made from a "Dashi pack" (kelp and bonito). I added mirin, sake and light colored soy sauce 薄口醤油 and let it come to gentle boil for few minutes, let it cool to room temperature and then refrigerated. You could use diluted "Mentsuyu"noodle dipping sauce 麺つゆ from a bottle as well.
To serve, I could have just seasoned with soy sauce or salt and used it as a topping for salad, noodles or a rice dish. The way I served the eggs is a good one if you just want to enjoy onsen eggs. I was a bit worried my wife might reject the looser texture of the egg white but she said although soft it was cooked not raw. She really liked the dish especially the custard-like texture of the egg yolk.
Compared to soft boiled or poached eggs, onsen eggs are easier to keep and serve. Once made, they can be kept in the refrigerator safely for, at least several days, and, since the white is not attached to the shell, the eggs slip out easily by just cracking the shell open (No more struggling to get the shell off soft boiled eggs).