Step 1

Place dashi-kombu in medium saucepan. Add 4 cups water and let soak 2 hours. Remove dashi-kombu; reserve dashi-kombu broth.

Step 2

Mix dashi, soy sauce, and mirin in small saucepan. Bring to simmer. Add 1 teaspoon bonito flakes. Strain sauce into medium microwave-safe bowl. DO AHEAD Dashi-kombu broth and dashi dipping sauce can be made 1 day ahead. Cover separately and chill.

Step 3

Place green onion in small bowl. Add enough water to cover. Let stand 5 minutes, then drain well. Bring dashi-kombu broth to simmer in medium saucepan over medium heat. Add mushrooms and simmer until almost tender, about 3 minutes. Add tofu and simmer until heated through, about 2 minutes. Add mizuna and simmer until just wilted, about 1 minute. Rewarm dashi dipping sauce in microwave. Divide between 2 shallow soup bowls. Using slotted spoon, divide tofu, mushrooms, and mizuna between bowls. Sprinkle green onion and ginger over and serve.