I recently had the pleasure of visiting the thousand-year-old Dogo Onsen, a hot spring in Matsuyama, Ehime. (Full disclosure: My travel and lodging were paid for as part of a press trip sponsored by the Tokyo Convention & Visitors Bureau.) At Funaya, the nearby ryokan where I stayed, I was served an onsen egg as part of my breakfast. It came in a small dish, with a chilled soy-dashi broth poured over it—one of the most common ways to serve onsen eggs, though by no means the only one. It's possible the chef cooked the eggs in the spa waters, since the hotel has the spa's thermal water piped in for on-site use, but it's just as possible that they were cooked using an immersion circulator, or even a simple pot of water heated to the right temperature.