Onsen tamago (Japanese: 温泉卵 or 温泉玉子, lit. 'hot spring egg') is a traditional Japanese low temperature boiled egg which is slow cooked in the hot waters of onsen (hot springs) in Japan.
The egg has a unique texture in that the white tastes like a delicate custard (milky and soft) and the yolk comes out firm, but retains the colour and creamy texture. This special texture is the result of cooking it in a temperature high enough to solidify the yolk yet too low to solidify the white.