How to Make It
Place 8 cups water and mushrooms in a Dutch oven; bring to a boil. Reduce heat, and simmer 45 minutes. Add the remaining 1/2 cup water and kelp; bring to a boil. Remove from heat. Drain mixture in a colander over a bowl, reserving mushrooms for another use; discard kelp. Store dashi in an airtight container in the refrigerator.