MAKE THE MARINADE
Add the onion, garlic, soy sauce, mirin, brown sugar, and ginger into a small saucepot and bring to a boil.
Add 1 tbsp of this “slurry” mixture to thicken the sauce.
Bring back to a boil, and then simmer for 5 minutes to thicken.
Remove from heat, strain out solids, and set aside.