Put water in a pot and bring it to a boil. Once boiling, reduce the heat to low, add bonito flakes, and let it simmer for 3 minutes.
Turn off the heat and strain the mixture through a sieve lined with paper towels or a cloth (such as cheesecloth). Alternatively, if you don't mind having some fine bonito flakes remaining, you can simply use a fine-mesh strainer. Bonito dashi is now ready.
Crack egg into a small bowl and beat it. Trim the base of mitsuba, separate the leaves, and cut the stems into ⅘-inch (2cm) pieces.
Return the bonito dashi to the pot and bring it to a boil. Once boiling, reduce the heat to low, and add salt and light soy sauce.
Mix starch and water in a small bowl. Gradually add the dissolved starch to the pot, stirring to combine. Continue simmering briefly to eliminate the powdery texture of the starch.
Gradually pour the beaten egg into the pot, ensuring even distribution. Once you are certain the egg mixture has cooked, gently stir it.
Turn off the heat and then add the mitsuba leaves and stems.