How to Make It
Combine all ingredients in a medium heatproof glass bowl; cover with plastic wrap. Place over a medium saucepan of simmering water over medium-low. Cook 1 hour. Remove from heat. Let cool completely, about 30 minutes. Chill at least 8 hours. Pour through a fine wire-mesh strainer into a bowl; discard solids.
Premium Rishiri kombu, from Rishiri Island off the coast of Hokkaido, Japan, can be ordered from toirokitchen.com. Use the wonderfully savory leftover Fortified Soy Sauce anywhere you’d use soy sauce—it also makes an excellent substitute for fish sauce.