Sometimes life happens and you just don't have 20 minutes to make dashi from scratch. On those occasions, you can use Dashi Packet or Dashi Powder.
Gather all the ingredients. Most Japanese recipes would say to gently clean the kombu with a damp cloth. However, these days, kombu is quite clean, so just make sure it doesn't have any mold spots and it's ready to use. Do not wash or wipe off the white powdery substance, as it has lots of umami.
Make several slits in the kombu to release more flavor.
To Make the Cold Brew Kombu Dashi Ahead of Time (Optional)
Put the water and kombu in a large bottle and let it steep on the counter for 2-3 hours in the summertime and 4-5 hours in the wintertime. You can also cold brew the kombu dashi overnight in the refrigerator.
To Make the Awase Dashi
Add the kombu and water to a medium saucepan. If you have cold brew Kombu Dashi (previous step), add the Kombu Dashi and hydrated kombu to the saucepan.
Turn on the heat to medium low and slowly bring to almost boiling, about 10 minutes.
Meanwhile, clean the dashi by skimming the foam and debris from the surface.
Just before the dashi starts boiling gently, remove the kombu from the saucepan (discard or repurpose in other recipes—suggestions follow). If you leave the kombu in the saucepan, the dashi will become slimy and bitter.
Add the katsuobushi and bring it back to a boil again.
Once the dashi is boiling, reduce the heat, simmer for just 30 seconds, and turn off the heat.
Let the katsuobushi sink to the bottom, about 10 minutes.
Strain the dashi through a fine-mesh sieve over a bowl or measuring cup. Reserve the katsuobushi and repurpose it; see the suggested recipes that follow. The Awase Dashi is ready to use.
If you are not using the dashi right away, store the dashi in a bottle or mason jar and keep it in the refrigerator for 3-5 days or in the freezer for 2 weeks.
What to do with the spent katsuobushi and kombu?
Save the spent kombu and katsuobushi in an airtight container and store it in the refrigerator for a week or in the freezer for up to a month.