Put all the ingredients in a small pot and bring to a boil.
Quickly turn the heat down to low (the sauce should still be bubbbling) and cook for 15-20 minutes, until desired consistency – the longer you cook the sauce, the thicker it will get. Also, keep in mind that it will continue to thicken as it cools down. I personally stop cooking it at around 17 minutes.
Transfer to a container and refrigerate it. Drizzle a little over sushi, serve with eel or grilled meat or chicken.