If you had to guess the food that inspires such rhapsody among chefs who have pretty much seen it all, stuffed mushrooms probably wouldn't come to mind.
To make this dashi, simmer kombu (a dried seaweed) and bonito (dried tuna belly flakes). (Both ingredients are available at most Japanese markets and some specialty stores.) If you can't find kombu, Ms. Schwertner suggests using dried shiitake mushrooms, which impart a similar umami flavor.