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. Add the kombu and water to a medium saucepan.

Turn on the heat to medium low and slowly bring to almost boiling, about 10 minutes. Heat it until tiny bubbles start to appear around the edges. (Don't let it boil.) Just before the dashi starts boiling gently, remove the kombu from the saucepan (save for 2nd use). 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

Optional: Niban (Second) Dashi
In a medium saucepan, put 6 cups of water and previously used kombu from making the first dashi. Remove the kombu just before the liquid comes to a boil.

Add used katsuobushi, bring it to a boil over medium-low heat. then lower the heat, and cook for 10 minutes, stirring occasionally. Turn off the heat.

Let the katsuobushi sink to the bottom and strain the dashi through the fine-mesh sieve.

To Store
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.