Here's what you do: put eggs in a basket or mesh bag, dunk in hot spring (70 degrees Celsius), wait 15-20 minutes. Easy! (If you are at Jozankei, we recommend spending the waiting time with your feet in the neighboring 足湯--foot bath.)
My friend, Shirley eagerly waits to see how they will turn out.
Done! (Heh heh... We cooked our eggs in an onion bag...)
Cello calluses make handling hot eggs easier.
15 minutes gives you quite a soft egg; we did ours for 18 minutes, and they were on the soft side of hard boiled. I imagine 20 minutes would be completely hard boiled.
I ate my eggs with ume-flavored dashi and soy sauce.
Keith preferred to eat his egg right out of the shell, topped with a little soy sauce.
Of course, some of you don't have access to special hot springs for cooking eggs, but you can get a similar result from cooking you eggs in 70 degree water on the stove, although I imagine the temperature would be hard to control.
And if you're worried about food poisoning because of low temperature, guess what? Japanese eggs are pasteurized. I assume you can get pasteurized eggs in some other countries too. But be careful, okay?