Step 1

In a small bowl, cover wakame with enough cold water to cover and let stand at room temperature until soft, about 15 minutes.

Step 2

Meanwhile, in a medium saucepan, combine kombu with 3 1/2 cups (830ml) cold water and cook over medium heat until water just starts to boil, about 6 minutes. Remove kombu with tongs and discard or reserve for another use. (Kombu can be cut into thin slivers and tossed into a salad.)

Step 3

Add a few tablespoons cold water to saucepan to lower temperature, then add katsuobushi and bring to a boil over high heat. Reduce heat and simmer dashi for 7 minutes.

Step 4

Strain dashi through a fine-mesh strainer set over a heatproof bowl; discard katsuobushi. Return dashi to a clean saucepan. Drain wakame and add to dashi along with tofu; be very gentle, since tofu is fragile and will break easily. Set over medium heat until warmed through.

Step 5

In a small bowl, whisk both misos with just enough warm dashi to dissolve into a thin paste. Remove dashi from heat and very gently stir in miso paste, being careful not to break the tofu.

Step 6

Carefully ladle soup into warmed bowls, garnish with scallions, and serve immediately.