Eel sauce, also called Unagi sauce, is a thickened and sweetened soy sauce that is perfect for dipping or drizzling. It doesn't actually contain any eel products and is a sweet and sticky sauce perfect for topping any fish or sushi.
In a small pot combine the soy sauce, sugar, and mirin.
Bring to a boil over medium-high heat. Reduce to medium-low and let simmer for 5-10 minutes, until reduced and thickened.
In a small bowl mix the cornstarch and water together to make a slurry.
Add the slurry to the sauce and stir until combined.
Cook for an additional 2-3 minutes until the sauce thickens enough to coat the back of a spoon. You may continue to reduce if you would like a thicker consistency.