Place all of the ingredients in a small saucepan over medium heat. Stir until the sugar is completely dissolved. Bring the mixture to a simmer, then reduce the heat to low. Allow the sauce to simmer gently for 10-15 minutes, stirring occasionally, or until the sauce has reduced by nearly half and has thickened to a syrup-like consistency. Remove it from the heat and allow it to cool completely. It will continue to thicken slightly as it cools. Transfer the eel sauce to an airtight jar and store it in the refrigerator where it will keep for at least 2 months.