In an ungreased frying pan, toast the sesame seeds on medium heat until toasty and fragrant. Shake the pan frequently so the sesame seeds don’t get burnt.
Transfer to the Japanese mortar and pestle (Suribachi and Surikogi). Grind until 90% of the sesame seeds are crushed and keep 10% uncrushed.
Discard the seeds from the kabocha and cut into ½ inch (1.3 cm) thick slices. Be careful as kabocha is quite tough to cut.
Cut the slices into a small chunk, roughly 1-inch pieces. Transfer the kabocha into a medium pot.
Add 4 cups water (or dashi, if you are not using dashi-included miso) and bring the water to boil over medium heat. Cover the lid leaving a small gap to prevent from boiling over.
Once boiling, skim the foam and scum on the surface using a fine-mesh skimmer. Reduce the heat to medium-low heat and cook kabocha for 15 minutes, or until tender. Do not overcook as kabocha will break into pieces.
You can check the kabocha’s doneness by inserting a wooden skewer into the kabocha. Check frequently to prevent it from overcooking.
When kabocha is ready, remove the bottom of shimeji mushrooms and add to the soup. Cook for 1-2 minutes until tender.
Turn off the heat and add 3 Tbsp miso first. Taste before adding another Tbsp of miso. You may not need to add more if it's flavorful enough to your taste.
Add the ground sesame seeds and mix well. Serve the miso soup in individual bowls and enjoy!