Brush any dirt or sandy bits off the dried kombu, being careful to keep the white ‘bloom’ that you might see on the surface. Soak the kombu in the distilled water for 8 hours or overnight.
Transfer the water and the kombu to a large saucepan and bring it to a simmer. Add the bonito flakes and keep the temperature at a low simmer. When the bonito sinks halfway down the liquid after about 5–10 minutes, strain the liquid through a fine sieve and allow the liquid to cool.
Store in a clean jar or airtight container in the refrigerator, or freeze for later use.
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