Place all ingredients in a small saucepan. Bring to a boil, whisking often. Remove from heat and serve.
Recipe Notes
If you can't find mirin or you want to avoid alcohol, substitute 1 tablespoon of rice vinegar and 1 tablespoon of water for the mirin.
You can adjust the thickness of the sauce by increasing or decreasing the amount of water. 1-2 tablespoons is nice for glazing. You may wish to thin it out for drizzling.