Add the dashi to the saucepan. If you are using dashi from the refrigerator, bring it to a slow boil (205°F/96°C) over medium heat and turn off the heat.
Add 4–5 Tbsp miso. Put the miso in a ladle, slowly add the dashi into the ladle, and stir with chopsticks to dissolve completely.
Cut 7 oz soft/silken tofu (kinugoshi dofu) into ½-inch (1.3 cm) cubes and add to the miso soup. Tip: Add the tofu after the miso is completely dissolved; otherwise, you might break the tofu when stirring in the miso.
Add 1 Tbsp dried wakame seaweed and the chopped green onions to the pot right before serving to keep their fresh fragrance and color. Tip: If you worry about salt intake, I recommend rehydrating the dried wakame in a separate bowl of water to get rid of the saltiness, instead of rehydrating it in the soup itself.
Serve immediately. Place on the right side of the table setting; you can read about this in my post Ichiju Sansai (One Soup Three Dishes).
In general, it‘s best to consume all the miso soup right away because it will lose its aroma and taste as time passes. Let your miso soup cool to room temperature (up to 4 hours; any longer and it will spoil) and then refrigerate. Keep for up to 2 days in the refrigerator. If you want to make a big batch to store for later, it‘s best to refrigerate the soup without adding the miso. When ready to use, add the miso only for the portion you need. You can freeze miso soup for up to 2 weeks. However, you have to remove the tofu before freezing as the texture will change.
If reheating, warm up the miso soup until it is just hot. NEVER BOIL miso soup because it loses flavor and aroma.
