In a medium-sized saucepan, add soy sauce, maple syrup, rice vinegar, garlic and ginger, chili sauce, sesame oil (if using), and black pepper.
Stir and bring to a simmer over medium heat.
In the meantime, make the cornstarch slurry. In a small bowl whisk cornstarch with cold water until dissolved.
Lower the heat and slowly pour the cornstarch slurry into the sauce, stirring constantly.
Cook for 2-3 minutes, or until thickened to your desired consistency and the sauce coats the back of a spoon.
If you prefer spicy bbq sauce, add red pepper flakes at this point and stir them into the sauce.
Once ready, remove the sauce from the heat and let it cool down.
Then, add the green onions (if using) and stir.
Pour the Korean BBQ sauce into a jar or an airtight container, and either use it right away or store covered with a lid for later.
