In a large mixing bowl, combine soy sauce, water, and sugar. Whisk until sugar dissolves. Add smashed garlic cloves and ginger slices.

Add chicken thighs to the marinade, ensuring they are well-coated. Seal with plastic wrap or in a resealable bag, removing air. Marinate in the refrigerator for at least 24 hours.

Preheat the grill to high heat (400–450°F). Allow marinated chicken to reach room temperature for about 20 minutes.

Remove chicken from marinade and grill for 5-7 minutes on each side until cooked through and internal temperature reaches 165°F.

Prepare a small pan with teriyaki sauce on the grill. Transfer grilled chicken to pan, cover, and steam in sauce for 10 minutes.

Remove from grill, let rest, slice, and garnish with chopped green onions and sesame seeds. Serve with jasmine rice or grilled vegetables.