1 cup lemon juice or combination lemon-lime (in Japan, sudachi citron or other very acid citrus fruits are used)
1/3 cup plus 2 tablespoon rice vinegar
1 cup dark soy sauce
2 tablespoon tamari sauce
3 tablespoon mirin, alcohol burned off
1/3 oz (10 g; 1 small handful) dried bonito flakes (hana-katsuo)
2 in (5 cm) square giant kelp (konbu)
Mix all ingredients and let stand for 24 hours. Strain through cheesecloth and mature 3 months in a cool dark place, or refrigerate. Keeps indefinitely, but should be used within 1 year for best flavor.
Makes 2-1/2 cups.