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 a few 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 (2 cm) 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.
In a small bowl, mix the starch with water until smooth, then gradually add it to the pot while stirring constantly to thicken and avoid lumps. Continue simmering for about 20 seconds to eliminate any powdery taste.
Gradually drizzle the beaten egg into the pot, distributing it evenly. Once the egg has set, gently stir it.
Turn off the heat and then add the mitsuba leaves and stems.