1. Rinse the kombu gently under cold water to remove any dirt.
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2. In a pot, add the water and the kombu. Let it soak for about 30 minutes.
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3. Place the pot over medium heat and slowly bring to just before boiling (around 175°F or 80°C). Do not let it boil, as this can make the broth bitter.
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4. Just before it starts boiling, remove the kombu and set aside.
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5. Bring the water to a gentle boil, then add the dried bonito flakes.
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6. Reduce heat and let it simmer for 2 minutes.
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7. Turn off the heat and let the bonito flakes settle at the bottom (about 2–3 minutes).
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8. Strain the broth through a fine mesh sieve or cheesecloth to remove the bonito flakes.
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9. Your homemade dashi is ready to be used immediately or stored in the refrigerator for up to 2 days.