My husband loves onsen tamago. I tried many times before but failed. I used thermometer this time and finally I succeeded.
Take eggs out from refrigerator and let them to be room temperature.
In a deep pot, pour into enough water that should covers eggs.
When the water temperature reaches 60 degree C, put the eggs into water gently.
Make sure temperature between 60 – 70 degree C. And boil for 30-40 mins. I boiled 37 mins. If the temperature gets higher than 70 degree C, add in a little cold water. You can also put in ice cube. Turn to the low heat or lowest heat of stove, so the temperature can be easily controlled.