In a medium saucepan, cover the bass and kombu with the water and bring to a simmer over medium heat. Cook the bass, skimming any foam that rises to the surface, until cooked through, about 10 minutes.
Using tongs, transfer the fish to a work surface and discard the kombu. Remove the pan from the heat and stir in the bonito flakes. Let the broth stand for 10 minutes. Meanwhile, using a fork, flake the meat from the bass into ½-inch pieces, and discard the skin and bones.
Pour the broth through a fine sieve set over a bowl and discard the bonito flakes. In a small bowl, stir the miso with 1 cup of the warm broth until dissolved. Pour the miso back into the saucepan with the remaining broth. Season with salt and keep warm over medium-low heat.
To serve, divide the scallions and fish among serving bowls and ladle over the warm broth. Garnish with shiso blossoms, if using, and serve while hot.
