Make the dashi stock by combining the water and kombu in a pot. On medium low heat, very slowly bring the dashi to a boil (will take 15-20 minutes). Just before the dashi starts to boil, remove the kombu.
Add the shiitake mushrooms and ginger (and any additional options). Simmer for a few minutes.
Remove a ¼ cup of the stock from the pot, place in a small bowl and add the miso. Using a fork, dissolve the miso and then add back to the soup.
Gently simmer for one minute. Do not bring to a boil, as this will damage the beneficial properties in the miso.
Serve garnished with green onions.