Wipe the kombu with a damp cotton cloth or paper towel. In a large pot, bring the water and kombu almost to a boil over medium heat. This should take about 10 minutes. Immediately before the water reaches a boil, remove the kombu, and either discard it or save it for preparing “second fish stock.” Use the liquid (kombu dashi) as a vegetarian stock, or follow the remaining instructions to make ichiban dashi (“first fish stock”).

Immediately after removing the kelp, add the katsuobushi (bonito flakes) all at once. Wait 10 seconds or until the liquid comes to a boil. Then turn off the heat, skim off any foam, and let the mixture stand for 2 minutes.

Strain the stock through a sieve lined with a tightly woven cotton cloth. Discard the bonito flakes, or save them with the kombu to make second fish stock.

Refrigerate the stock, tightly covered, for up to four days, or freeze it into cubes in an ice tray.