Put water and dashi packet in a medium pot. Turn on the heat to high.
While you wait for the water to boil, slice tofu into small cubes and prepare your green onion.
When water is boiling, bring heat down to medium and simmer for 3 minutes.
Remove dashi pack from the water and discard. Add tofu to your pot of dashi. Turn off the heat. Your dashi should still be very hot. If you added a lot of cold tofu, you may need to keep your dashi on the heat until it comes back up to almost boiling. See notes below for what to do if you add other ingredients. In any case, turn off the heat before you move on to the next step.
Dissolve 3 tbsp of white miso paste into your dashi. Taste your soup and add more miso as needed.
Scoop miso soup into small bowls (one serving is a little under one cup) and garnish with the chopped green onions. Serve immediately. Leftover miso soup can be kept in the fridge for up to two days, but it tastes best fresh.
Notes
I highly recommend the Kayanoya brand for dashi packets. If using a different brand, follow the instructions on the package for how to make dashi.My favorite brand for miso is Hikari, but you can use any miso you like. Feel free to substitute red or other types of miso as well, but know that the miso flavor may be much stronger or lighter depending on what you use. If you are adding raw vegetables or other additions to your miso soup in step four, bring your dashi back up to a simmer until your vegetables cook through. For example, for frozen kabocha, this may take up to 5 minutes. See blog post above for other ideas on what to add to miso soup.