Onsen tamago, a softly cooked egg in flavorful soy broth, is a popular breakfast item in Japan. Traditionally, it's made by cooking the eggs in hot geothermal spa water, but it's just as easy to make it at home. All you need are a pot and a digital thermometer. Here's how to do it.
Why It WorksJapan's spa treatment for an egg.
- Cooking the eggs at a carefully controlled below-boiling temperature produces the silkiest, most custardy eggs, with tender whites and warm, runny yolks.
- A mixture of soy, mirin, and sugar is blended with dashi to create a deeply flavorful broth in no time.
For the Eggs, If Using an Immersion Circulator: Following manufacturer's instructions, preheat water bath to 167°F (75°C). When water is ready, add shell-on eggs and cook for 13 minutes. Transfer to an ice bath to chill. Eggs can be refrigerated, shell on, for up to 2 days.
For the Eggs, If Using a Pot and an Instant-Read Digital Thermometer: Fill a large pot with water and bring to 167°F (75°C). Add shell-on eggs and cook for 13 minutes, adjusting heat as necessary to maintain temperature. Alternatively, transfer hot water to a cooler, add eggs, and cook for 13 minutes, using extra boiling or cold water to adjust heat to maintain 167°F. A small fluctuation of a degree or two up or down is okay; just do your best to maintain the temperature the entire time. Transfer eggs to an ice bath to chill. Eggs can be refrigerated, shell on, for up to 2 days.
For the Soy-Dashi Broth (optional): While eggs cook, bring mirin to a simmer over medium heat in a small saucepan. Add sugar and stir to dissolve. Add soy sauce, stir well, and bring to a simmer. Remove this kaeshi mixture from heat and transfer to refrigerator to cool.
When ready to serve, combine 1/4 cup kaeshi (soy-mirin) mixture with 3/4 cup dashi. Any additional kaeshi and dashi can be reserved for another use. (Mixed together, as in this recipe, they make an excellent cold dipping broth for chilled soba or udon noodles.)
Working with one at a time, carefully crack eggs and peel off enough of the shell to slide the egg out into a small mixing bowl. Using a spoon and/or a clean paper towel, carefully separate soft-cooked egg from any loose whites. Slide each egg into a small serving bowl, pour broth around it, and garnish with scallions. Serve.