Eel sauce, also called unagi sauce or unagi no tare in Japanese, is based on a Japanese condiment nitsume which contains eel broth, but mine one does not. This thick sauce is easy to make at home and can be brushed on eel or drizzled over sushi rolls.
Add the sugar, soy sauce and mirin to a small saucepan. Whisk to combine.
Cook, stirring occasionally over medium heat until the mixture thickens and has reduced to about ¾ cup (253g). Note that it will thicken more as it cools. It should be the consistency of honey.
Let cool completely and use or store refrigerated in a resealable container for up to 2 weeks.