Shimeji mushrooms, onion, and purified water should be combined in a medium pot. For instructions on how to extract the taste of shimeji, see the "Notes" section below. Cook covered over medium heat until a gently boil. Then turn the heat down to low and keep simmering for an additional five minutes.

Turn the heat off. the soup with miso. To do this, put some miso on a ladle and dip it a little bit into the soup. To help the miso dissolve, slowly pour the soup into the ladle using chopsticks. Reset the heat to medium-low, and cook the food for a further few minutes to reheat.

Stir and top with scallions before serving.