Put 400 ml of water in a small pan and bring to a boil.
Pierce the rounder end of the egg with a thumb tack. See Recipe ID: 1528080 for details.
Turn the heat off under the Step 1 pan when the water has come to a boil. Add the pierced, still-cold eggs from Step 2.
The eggs will stick out a little bit from the water, but it's not a problem. Cover with a lid and leave it to cook for 10 minutes.
I measure the water temperature during the 10 minutes. After 3 minutes: 83°C. After 5 minutes: 78°C. After 8 minutes: 74°C. After 10 minutes: 71°C.
After 10 minutes, take the eggs out immediately and run cold water over them. Leave them in cold water until they are cooled down completely.
I used medium sized eggs. The ambient temperature in the room was 27°C though, so the water may cool slower or faster if it's hotter or colder.
Keep the heat conductivity of the pan in mind too. I used a 15 cm diameter small pan and kept it on the stove top. Aluminum pans conduct heat better, so the hot water will cool faster.
Note: According to new food and nutrition guidelines, a size M egg is 58 to 64 g. A size L egg is 64-70 g. Use "M" eggs.
Note: Here I used 64 g eggs with success! After 10 minutes the water temperature was 71°C. This was an ungraded egg but again, to be redundant, please use size "M" eggs.
For L or LL (extra-large) eggs: They took 11 minutes when the room temperature was 27°C again. The egg on the left is an extra-large (LL) that weighs 74 g. The one on the right is L size and weighs 69 g. Leave the eggs in the hot water until they have set as much as you like.
To list the times by size, using 400 ml of water covered with a lid. Size M: 10 minutes; size L: 11 minutes; size LL: 12 minutes. But it varies depending on the room temperature. (continued in Step 13)
The times listed in Step 12 are for when the room temperature is 27°C in the summer. In May, when the room temperature was 23°C size M eggs (62 and 63g) took 12 minutes.
Story Behind this Recipe
Conventional wisdom says that you need to use room temperature eggs... But onsen soft-boiled eggs are made by taking advantage of the different set points between the yolk and white of an egg, so if you add still-cold eggs straight from the refrigerator, the water temperature goes down, which is good. The important thing is to maintain a water temperature above 70 °C. I experimented.
Since cold eggs are used, I pierced the shells beforehand to prevent them from cracking. This is quite easy to do. Then just put the eggs in the hot water and set the timer! Make sure to measure out the water properly and to start the timer after putting the lid on, since the ambient temperature can affect the results. In addition, try to use the same pan every time for consistent results.