Make Dashi broth according to Dashi recipe. Or you can use an instant Dashi.
Cut kabocha into 1/3" thick bite size pieces. Cut the bottom of shimeji mushroom and separate into pieces.
Cook the kabocha in the Dashi broth covered for 3 minutes, and add the shimeji mushroom and cook for 2 more minutes.
Dissolve the miso paste in the soup.