In a small pot, combine dashi, soy sauce, sake, and a pinch of salt. Bring the mixture to a gentle boil over medium-high heat, stirring occasionally, about 3–5 minutes.

Once boiling, reduce heat to a simmer and mix potato starch or cornstarch with a few tablespoons of cold water until smooth. Slowly pour this into the pot while stirring gently. Allow to thicken for about 1–2 minutes.

Drizzle the whisked egg over the chopsticks held vertically in the center of the pot. Let it cook for about 20 seconds without stirring.

Add baby spinach to the pot, stirring delicately to wilt for about 1 minute.

Remove from heat, ladle into bowls, and garnish with soy sauce or green onions if desired.