In a bowl, whisk together 1 clove minced garlic, 1/8 tsp freshly grated ginger, 1 tbsp brown sugar, and 1/3 cup low sodium soy sauce.
Add the chicken thighs to the soy sauce mixture and stir just to combine. Cover the chicken with plastic wrap and let it marinate in the refrigerator for at least 2 hours.
Once the chicken has marinated, fire up the grill to medium-high heat and cook each chicken thigh for about 4 minutes on each side.
While the chicken is grilling, heat 2 tbsp of grapeseed oil in a large pan over medium-high heat. Add the minced garlic and stir for about 10 seconds. Then add in the soy sauce, sake, mirin, brown sugar, and water. Let the sauce come to a simmer, then reserve 1 tablespoon of the liquid into a small bowl. Then add 2 tablespoons of cornstarch into that bowl. Mix until the cornstarch is dissolved. Then pour the mixture back into the sauce. Bring the heat down to low and let it simmer for about 4 minutes. You don't want it to cook for too long, or else it will become bitter.
At this point, you can cut the chicken into strips and serve over hot rice. Then pour the sauce over the hot rice and chicken.