Long before sous vide and immersion circulators, before instant-read digital thermometers, before temperature-stable combi ovens, before any of the modern gear and techniques that we use to cook something as simple as an egg at consistent sub-boiling temperatures, there was Japanese onsen tamago. Onsen, in Japan, refers to the hot geothermal springs throughout the country, as well as to the spas where visitors can bathe in them; tamago, meanwhile, is the word for "egg." It so happens that those temperature-stable spa waters have just about the perfect level of heat for making soft-cooked eggs. After dropping shell-on eggs into those spring waters, Japanese people could leave them unattended for a few hours and come back to find the silkiest, most custardy eggs imaginable.
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.
And that's the beauty of onsen eggs: You don't actually need an onsen to make them.
The easiest method is to use an immersion circulator, which, like the onsen waters, can hold precise temperatures for as long as you need. You can, for instance, set the circulator to 145°F (63°C) and let the eggs cook for anywhere from 45 minutes to one and a half hours. Because time is a factor in the gelling reactions that cause egg whites and yolks to thicken and set, you'll notice changes in the egg within that time frame. At 45 minutes, the whites will be softly set, and the yolk will be warm but runny; once you hit an hour and 30 minutes, the whites will be more or less the same, but the yolk will have thickened to a custardy texture. Either is good, depending on what you want (and, if you want a lot more help deciding exactly what you want, be sure to read Kenji's in-depth guide to sous vide–style eggs.)
The advantage of using such a low temperature, though, is also its disadvantage. It takes time. Forty-five minutes is often longer than most of us are willing to wait for a soft-cooked egg. Thankfully, there's an even easier and quicker method that splits the difference between such a low-and-slow approach and full-on boiling water. Popularized by Aki Kamozawa and Alex Talbot in their book, Ideas in Food, the method calls for cooking the eggs at 167°F (75°C) for just 13 minutes. The higher temperature speeds up cooking, allowing for a shorter cooking time, and creates a gentle temperature gradient within the eggs, with slightly firmer whites transitioning to warm, runny yolks in the center. It's worth noting that if you let the eggs go much longer than 13 minutes, you'll end up with much harder-cooked eggs with solid yolks, which isn't the goal here.
If you have an immersion circulator, you can set it to 167°F, get a timer running for 13 minutes, and walk away until it beeps. You can also use our beer-cooler sous vide method: Fill up a small insulated cooler with water heated to a couple of degrees above 167°F, to account for the temperature drop the eggs will cause (use a kettle of boiling water and a jug of cool water to fine-tune the temperature); drop in the eggs; seal the lid; and start your timer.
Don't have a circulator or a cooler handy? You can still make these eggs, though it requires 13 minutes of babysitting. All you have to do is bring a pot to 167°F, keeping an eye on the temp with a digital thermometer; lower the eggs in; and adjust the heat as needed to keep the water hovering around that 167°F mark.
It doesn't matter much if you go a couple of degrees above and below that mark during the 13 minutes, as long as you keep the temperature as close to that as possible. As you can see in the above photo, my water wasn't always at precisely 167°F, and I still got great results.
When the eggs are done, just transfer them to an ice bath to chill, then peel them (or hold them in the fridge for a day or two until you're ready to use them). The peeled eggs will have some looser whites, which is just the watery portion you see when they're raw. You can remove them with a clean paper towel or scoop the egg from them with a spoon.
Onsen tamago are great served in noodle soups, on top of rice, or on dishes like gyudon, but I like the simplicity of the way they were served at the ryokan.
I've found various recipes for the broth for onsen tamago online and in cookbooks, and they all agree on one thing: It's the classic Japanese mixture of dashi (Japanese smoked-bonito and kelp broth), soy sauce, and mirin, with sugar for just enough sweetness to balance the other flavors. The trickier part was figuring out how to combine them.
One source I read said that the ideal formula was an 8:1:1 ratio of dashi to soy and mirin, but this notion of an "ideal" ratio was undermined by several other books and websites I consulted, which suggested different ratios based on the use. I tested out that 8:1:1 ratio, which was mild and pleasant if unexceptional, as well as the 4:1:1 ratio another source recommended, which I found a little too heavy on the alcoholic punch of mirin.
Ultimately, I found my solution in Nancy Singleton Hachisu's book, Japanese Farm Food. In it, she first makes a 4:1 mixture of soy sauce and mirin, heating the mirin to cook off its alcohol, then dissolving sugar into it and finally cooking it all briefly with the soy sauce. She calls this mixture kaeshi, and it's this that she blends with the dashi, in a 3:1 ratio of dashi to kaeshi when serving it chilled. (She uses a 10:1 ratio when serving the broth hot.) The result is a broth that's more complex, with the mirin acting as a subtle background note. This method is especially cool because you can make a larger batch of the kaeshi and keep it in the fridge—in her book, Hachisu says up to a month, though I've found that it typically lasts even longer—then mix it with dashi whenever you need it. It's a condiment I will definitely be keeping in my kitchen from now on.
When I emailed Hachisu and asked her about the kaeshi, she told me it was a method she'd picked up from a Japanese chef named Kanji Nakatani, or "Kanchan" for short, who runs a couple of top-notch soba restaurants near her farm in Saitama Prefecture. The ratios he uses, she said, are just guidelines, and can be adjusted for any number of reasons, including the season—in the summer, for instance, he may make the broths slightly saltier. Most of us won't be quite so expert as to fine-tune the broth with those kinds of considerations in mind, but it's helpful to be reminded that the broth can be adjusted to your own taste and mood, maybe with more dashi for a lighter flavor one day and more kaeshi for a more salty-savory kick another.
To serve the onsen eggs, simply slide them into small bowls and pour the chilled broth around them. Top with thinly sliced scallion, and you're all set. It's not quite the same experience as being at an onsen in Japan, but it'll get you closer.