If you’re not familiar, an onsen egg, while traditionally cooked in a hot spring, is also classified by its barely set white and gelled yolk.
In Kakuriyo -Bed & Breakfast for Spirits-, Aoi is treated to an onsen tamago with some freshly bottled carbonated water by Odanna, the Ogre lord of Tenjin-ya.
It means “hot spring egg,” which you’re probably thinking is a fancy name for a special kind of egg dish, but actually, it just means that it’s an egg cooked in a hot spring.