Fish Miso soup

Name: Fish broth Miso soup
Prep time: 10 min
Cook time: 20 min
Total time: 35 min
Yield: 5-6
Ingredients: Fish heads and bones: 500g; Green onion: 1; Miso paste: 3-4 tablespoons; Kombu (dried kelp): 1 piece (10cm square); Water: 6 cups (about 1.5 liters); Salt: a dash