In a small saucepan, combine soy sauce, mirin, and sake.
Add sugar and stir until dissolved over medium heat.
In a separate bowl, mix cornstarch with water until smooth (if using).
Once the mixture begins to simmer, add minced garlic and grated ginger.
Stir continuously for about 2 minutes to blend flavors.
If using cornstarch, slowly add it to the saucepan while stirring.
Cook for another 2-3 minutes until the sauce thickens.
Remove from heat and let cool slightly.
Transfer to a jar or bottle for storage.
Garnish with sesame seeds and chopped green onions before serving.