PreparationPrepare wakame: Combine wakame with warm water to cover by 1 inch and let stand 15 minutes, or until reconstituted. Drain in a sieve. Make soup: Stir together miso and 1/2 cup dashi in a bowl until smooth. Heat remaining dashi in a saucepan over moderately high heat until hot, then gently stir in tofu and reconstituted wakame. Simmer 1 minute and remove from heat. Immediately stir in miso mixture and scallion greens and serve.