Instructions

1. Squeeze the juice from 1 lemon until you have 2 tablespoons. Squeeze the juice from 1 to 2 limes until you have 2 tablespoons. Place the lemon juice, lime juice, 1/4 cup plus 2 tablespoons soy sauce, 1/4 cup mirin, and 2 teaspoons rice vinegar in a resealable container or small jar. Add 1 (3-inch square) piece kombu and 1/4 lightly packed cup katsuobushi. Stir to combine.
2. Seal the container and let steep for 4 hours at room temperature or overnight in the refrigerator. Strain through a fine-mesh strainer into another jar or bowl. Discard the kombu and katsuobushi, or reserve for another use.
3. Place 1/4 cup plus 2 tablespoons soy sauce, 1/4 cup mirin, and 1 (3-inch) piece kombu in a small saucepan over medium heat until bubbles start to form at the edges, about 2 minutes. Do not boil, or the kombu will create a slimy texture. Remove the saucepan from the heat.
4. Add 1/4 lightly packed cup katsuobushi, stir to combine, and let steep for 10 minutes. Meanwhile, squeeze the juice from 1 lemon until you have 2 tablespoons. Squeeze the juice from 1 to 2 limes until you have 2 tablespoons.
5. Strain the soy sauce mixture through a fine-mesh strainer into a resealable container or small jar. Add the lemon juice, lime juice, and 2 teaspoons rice vinegar, and stir to combine.