In a large soup pot over medium heat add the scallion white parts, ginger, garlic, and sesame oil. Cook for 1 minute and add 8 cups water. Rinse the kombu and add it to the pot along with the bonito flakes. Bring it to a simmer and cook for 10 minutes - do not let it boil. Remove the kombu and set it aside. Add the dried mushrooms and miso to the pot and let it simmer gently for 10 to 15 minutes, or until the mushrooms are hydrated and tender. Add the bok choy and simmer until it is tender, about 10 minutes. Add the tofu and cook for another 5 minutes. Ladle into bowls and garnish with the reserved green parts of scallions.