Combine water and kombu in a pot
Let the kombu soak in the water for 20 to 30 minutes
Heat the water slowly over medium-low heat
Remove the kombu just before the water begins to boil
Add bonito flakes if making awase dashi
Turn off the heat and let the bonito flakes steep for 1 to 2 minutes
Strain the stock through a fine sieve or cloth
Use the dashi immediately or refrigerate it briefly