Ponzu sauce

1 c. lemon juice, or lemon-lime, or sudachi citron or other very acid citrus fruit
1/2 c. rice vinegar
1 c. dark soy sauce
2 tbsp tamari
3 tbsp mirin, alcohol burned off
1/3 oz (10 g) hana-katsuo
2-inch square konbu

Mix all ingredients and let stand 24 hours. Strain, mature 3 months in a cool dark place or refrigerate. Keeps indefinitely, but is best used within 1 year.

Source: Japanese Cooking, a Simple Art, by Shizuo Tsuji, p. 172