Combine zests and salt on a cutting board. Chop, dragging the knife blade at an angle across mixture, until a coarse paste forms. Transfer to a small bowl. Mix in remaining ingredients. Transfer to a small jar, cover, and chill for 2 days. Can be made 2 weeks ahead. Keep chilled. Use as a condiment for nabemono (Japanese hot pot), miso soup and sashimi. Makes 1/2 cup.
Approximate Nutrient Analysis per teaspoon serving:
5 calories, 0 g fat, 0 g saturated fat, 0 mg cholesterol, 250 mg sodium, 1 g carbohydrate, 1 g fiber, 0 g sugar, 0 g protein