Let's cook.
In a medium saucepan, combine the dashi broth, soy sauce, mirin, and salt. Bring the mixture to a gentle simmer over medium heat.
While the broth is simmering, whisk the eggs in a small bowl until well combined.
In another small bowl, whisk together the cornstarch and water to create a slurry.
Once the broth is simmering, slowly pour the cornstarch slurry into the broth while stirring continuously to prevent lumps. Continue to stir until the broth slightly thickens, about 1 minute.
Reduce the heat to low. Slowly drizzle the whisked eggs into the simmering broth in a thin stream, stirring gently with a fork or chopsticks in a circular motion to create delicate egg ribbons.
Cook for 1-2 minutes, or until the egg is set. Do not overcook, as the egg will become tough.
Ladle the soup into two bowls and garnish with chopped green onions. Serve immediately.
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