Prepare the Eggs: Crack the eggs into a measuring cup or bowl with a spout and gently beat them, then set aside.
Season the Dashi: In a pot, heat the dashi broth over medium heat. Add soy sauce and sea salt, then taste and adjust the seasoning as needed.
Thicken the Soup: In a small bowl, combine the potato starch and water, mixing until smooth. Before the starch settles, pour the mixture into the simmering dashi while stirring from the bottom of the pot with a ladle. This creates a gentle thickness that helps form soft egg ribbons.
Add the Eggs: Bring the soup to a boil. Slowly pour half of the beaten eggs into the soup in a circular motion. Wait about 10 seconds until the eggs float and begin to set. Pour in the remaining eggs the same way and wait another 10 seconds, then gently stir from the bottom with a ladle. When the eggs are softly set and the soup becomes clear, remove the pot from heat.
Serve: Serve the soup hot, garnished with chopped green onion.