Takashi's Dashi

Place kombu and 9 cups water in a large pot; let soak at room temperature for at least 20 minutes and up to overnight.

Place pot over high heat and bring to a boil. Remove kombu and discard. Reduce heat to a simmer and gently stir in katsuobushi. Let simmer for 10 minutes. Strain through a fine mesh sieve into a 2-quart container.