Unagi Sauce

1.In a small saucepan, add mirin, sake, sugar. Turn on the heat to medium and whisk the mixture.
2.Then add soy sauce and bring it to a boil. Once boiling, reduce heat to low and continue simmering for 5 minutes. Toward the end of cooking, you will see more bubbles.
3.Turn off the heat and let cool. The sauce will thicken more as it cools. You can store the sauce in an airtight jar and keep in the refrigerator for up to 2-3 months.