Pour six cups of water into a pot and add the kombu and bonito flakes.
Bring to the boil, simmer for five minutes then turn off the heat. Leave to steep for 10 to 15 minutes then strain.
Pour the strained liquid (that's your dashi) into a clean pot.
Turn on the stove and pour in the soy sauce. Taste and add as much salt is needed to get a good flavor.
If you're adding extra ingredients to your egg drop soup (mushrooms, seaweeds or vegetables), add them now.
Stir in the starch solution. The broth will appear cloudy but, as it heats up, it will turn clear again.
Simmer the broth for about five minutes then turn off the heat.
Pour the beaten eggs in a thin stream over the entire surface of the broth.
Leave to let the eggs cook in the broth before stirring gently.
Ladle your Japanese egg drop soup into bowls, sprinkle in scallions and serve.