Wash chicken thighs thoroughly and place in a gallon size zippered bag. (Or you can use a bowl.) In a small saucepan, combine the soy sauce, mirin and sugar. Bring to a boil, stirring often to dissolve sugar. Set aside to cool. Reserve about ½ cup of sauce for serving. In a blender or mini food processor, combine the remainder of the cooled teriyaki sauce, ginger, garlic and sesame oil. Blend until mixture is fully combined. Pour mixture into bag or bowl with chicken, working the chicken so every piece is coated. Refrigerate for several hours or overnight. When ready to cook, spray a grill or grill plate with cooking spray. When hot, place chicken on grill. (Discard marinade.) Cook for several minutes on each side to gently char. When done, slice the chicken in strips. Serve with reserved teriyaki sauce over a bed of rice or salad. Garnish with thin slices of green onion.