Fill a medium sized soup pot with water and add kelp. Heat, uncovered, so as to reach the boiling point in 10 minutes. Important: Kelp emits a strong odor when boiled, so be sure to remove it from water before it boils. Insert your thumbnail into the fleshiest part of the kelp, if it is soft, remove it from the water. If tough, return it to the pot for 1 to 2 minutes. Keep it from boiling by adding 1/2 cup cold water.
Remove kelp and bring stock to a full boil. Add 1/4 cup cold water to bring the temperature down quickly and immediately add the bonito flakes. Bring to a full boil and remove from the heat at once. Allow the flakes to start to settle on the bottom of the pot. Skim foam and filter through cheesecloth lined sieve.
Recipe courtesy of David Rosengarten