
Make the Dashi. Place the water and kombu in a medium pot over medium heat, and bring to a simmer. Right before the water starts boiling, remove the kombu. Pro tip: boiling kombu can make the broth slimy or bitter, so make sure to not leave it in. Turn off the heat, stir in the bonito flakes, and let them steep for 3 to 5 minutes. Strain the liquid through a fine mesh sieve and into a bowl for fresh, clean dashi.
Rehydrate the seaweed. Place the wakame in a small bowl with warm water. Once it expands (about 30 seconds), drain and set it aside.
Make the miso soup. Return the warm dashi to the pot on medium heat. Add the tofu (and optional soy sauce and ginger, if you'd like), and simmer gently for 2 to 3 minutes.
Finish and serve. Remove the pot from the heat. Place the miso paste in a small bowl, whisk it with a ladle of hot broth until smooth, then stir it back in the pot. Stir in the green onions and wakame, then serve warm.