Add all the ingredients into a small saucepan and cook over medium heat. Once boiling, reduce the heat to low and let simmer for 10 minutes, or until the sauce has thickened.
Remove from heat and let cool to room temperature. Once cool, transfer to a container with a tight-fitting lid until ready to use.
Notes
Mirin: You can use sake or rice vinegar instead of mirin.Sugar: You can use a liquid sweetener like agave instead of white sugar.Adding flavors: Ginger is a great addition to this sauce. I like to add grated fresh ginger. I have also added minced garlic which goes well with grilled fish.Storage: Eel sauce can be stored in the refrigerator, covered, for up to one week. Give it a stir before using it, as there may be a little separation.Freeze: Or lace the sauce in a glass jar and store it in the freezer for up to two months.Makes about 1 1/2 cups.