COMBINE KOMBU AND WATER in a medium saucepan and bring to just under a boil, then remove from heat. Use tongs to remove and discard the kombu. Sprinkle the bonito flakes into the pan and let steep until they sink to the bottom, about 3 minutes. Strain broth through a fine sieve before using. Dashi can be kept in an airtight container in the refrigerator up to 4 days.
Kombu (also called kelp) is a type of sun-dried seaweed that has been processed into sheets. It should not be rinsed with water before using or it will lose some of its flavor; instead, lightly wipe it with a dry cloth. Unopened packages will keep indefinitely; after opening, store kombu in a resealable plastic bag in a cool, dry place and use within six months.
Bonito flakes are made by boiling, smoking, sun-drying, and then flaking fresh bonito, a type of small tuna. Store in an airtight container in a cool, dry spot for as long as a year (check the label for a freshness date).
