When it comes to creating a rich and balanced soup, the trick to keep in mind is always make sure you're developing layers of flavor. This starts with the broth and should be carried through to the garnishes. This miso soup begins with a simple kombu-based dashi (Japanese broth), then various flavors are added (soy sauce, ginger, Sriracha sauce, rice wine vinegar, and sake), followed by white miso, the main ingredients (tofu, shiitake mushrooms, kale, and sautéed scallions), and finally topped with a sprinkle of diced fresh scallions.
Add 2 quarts of water and the kombu to a large pot; let it soak for 30 minutes, then bring to a simmer for 10 minutes over medium-low heat. Remove the kombu from the water and discard, then take a cup of water from the pot and set aside. Add the soy sauce, sake, rice wine vinegar, Sriracha, and ginger to the remaining water and increase the heat to medium.
Meanwhile, whisk the miso paste into the reserved water until smooth, then pour the mixture into the pot and let everything cook for another 5 minutes. While the soup is cooking, sauté half of the scallions in a tiny bit of oil until crispy, then set aside.
Add the tofu, mushrooms, kale, and cooked scallions to the pot and cook just until everything is warmed through. Ladle the soup into bowls and garnish with remaining fresh scallions.