Onsen tamago is a Japanese method for poaching eggs in he shell.
Japanese owe this cooking method not to noodles, but to the country’s bountiful natural hot springs. Hot springs or “onsen,” dot volcanic Japan from tip to tip (dipping into a steaming onsen one of the great pleasures of visiting Japan), and a custom for cooking eggs at these springs evolved over the years — toss them into the hot water, wait a bit, and the egg magically poaches. The secret is the onsen’s water temperature, which causes the egg’s yolk and albumen congeal into a nice sphere on the outside, and beautifully creamy and tasty on the inside. It’s a great way to cook eggs if you’re a stone’s throw (or make that, an egg’s throw) from an onsen.
Interesting, but not very helpful. What about the rest of us? Harris has more
I asked Chef Rio and here is his easy technique: Fill a large pot with water and bring it to a boil. Turn off the heat and add 4 eggs. Leave the eggs in the water for about 30 minutes. Remove the eggs, crack them open and — viola! — you’ll have nicely poached onsen tamago. (For those of you who demand perfection, maintain the water temperature at exactly 145 degrees F (or 65 degrees C), which will yield an impeccably spherical poached egg.)