What is the Japanese soft-boiled egg cooked in hot spring water?

The Japanese soft-boiled egg cooked in hot spring water is onsen tamago.

Onsen tamago, literally “hot-spring egg,” is traditionally cooked slowly in the naturally warm water of a Japanese onsen. The low-temperature method produces an unusual texture: the white becomes delicate and custard-like, while the yolk remains creamy and softly set. The egg is cooked inside its shell and opened just before serving.

Traditional preparation uses water around 65–68°C for roughly 20 minutes, although exact times vary with egg size and temperature. At home, the same effect can be approximated with carefully controlled warm water. Onsen tamago is commonly served in a light dashi-based sauce, sometimes with soy sauce and chopped scallions.

It differs from ajitsuke tamago, which is usually boiled and then marinated in soy sauce, and from tamagoyaki, a rolled Japanese omelet. Chawanmushi is also soft and custard-like, but it is a steamed egg custard prepared with additional ingredients rather than an egg cooked in its shell.