Put the water in a pan, add the sliced mushrooms and simmer gently over low heat.
Just before it comes to a boil, add the green onions and/or tofu to taste.
When it comes to a boil, add the miso, turn off the heat and it's done.
Be sure to ladle plenty of mushrooms into each bowl.
One person tried it with wakame seaweed. Looks great!
Story Behind this Recipe
When I used to live in the U.S. and didn't have access to Japanese mushrooms like shimeji and shiitake, I used mushrooms instead in miso soup. It's actually pretty tasty! Since then it's become one of my favorite dishes to make.
Place the mushrooms in cold water (don't start boiling them with hot water) and simmer them very gently to extract their umami. If you are in a hurry, add a little dashi stock. Don't add too much miso since it will overwhelm the flavor of the mushrooms. Use plenty of mushrooms for the best result.