1 big piece rehydrated kombu (from making Vegetarian Dashi)
2 tablespoons soy sauce
1 tablespoon sugar
1 tablespoon mirin
1 teaspoon rice vinegar
1⅔ cups water
1 teaspoon sesame seeds
Cut the kombu into strips about 1½ inches wide, then julienne them. Add to a saucepan with the soy sauce, sugar, mirin, vinegar and water, and bring to the boil. Reduce the heat to medium—it should be between a simmer and a boil, and cook until the liquid reduces to a thick, sticky glaze.
Taste the kombu; it should be quite soft but not mushy. If it needs more cooking, add a little water. There should be no liquid left; it should be a glaze as opposed to a sauce. When the kombu is ready, stir in the sesame seeds. Leave to cool before using as a filling for onigiri, a topping for rice or on its own.