Step 1

Combine vinegar, mirin, and kombu in a small saucepan. Bring to a simmer. Remove from heat and stir in katsuobushi. Cover and let steep for 10 minutes. Strain through a fine-mesh strainer and discard solids (or reserve to make a second batch, using some additional katsuobushi in the second batch). Allow liquid to cool completely.

Step 2

Combine steeped mirin, citrus juice, and soy sauce. Ponzu will keep in a sealed container in the refrigerator for up to 1 week.