This Asari Miso soup (あさり味噌汁) contains only a handful of ingredients and yet the broth is ultra-flavorful, thanks to the Asari clams (Manila Clams).
The day before you plan to make your miso soup, add the clams and 1 teaspoon of salt to a bowl and add just enough water to cover the clams. This will make the clams open and release any sand they contain. The next day, rinse the clams
Add the mushrooms, clams and 2 cups of fresh water to a small saucepan and bring to a boil. Cook until the clams open (about 1-2 minutes).
Turn the heat down and add the miso, dissolving the paste in a ladle filled with a little soup first to avoid clumps. Because different types of miso vary in salinity, taste the soup and adjust the miso to taste.