I learned this dish many years ago from chef Hiro Sone. It is a fast way to make delicious ponzu and it can hold for up to a month when refrigerated. This sauce is so versatile, you can add a little to mayonnaise or eat it with raw fish, beef tataki, etc.
Combine all ingredients except the zest in a non-reactive medium-sized sauce pot, bring to a boil, turn off then add zest. Let rest 5 minutes, then strain and chill.