In a large mixing bowl, combine soy sauce, water and sugar. Stir until the sugar dissolves, then add the garlic and ginger. Reserve 1/2 cup in a clean covered container and refrigerate it before adding the raw chicken to the remaining marinade.

Place the chicken thighs in the remaining marinade and coat them well. Cover and refrigerate for at least 24 hours and no longer than 2 days.

Heat your grill to high heat to achieve a nice sear on the chicken.

Remove the chicken from the marinade, let excess liquid drip off and discard all marinade that touched the raw chicken. Grill for 5–7 minutes per side, until lightly charred and the thickest part reaches 165°F (74°C).

Pour the refrigerated 1/2 cup of reserved, unused marinade into a clean pot or grill-safe pan over medium heat.

Transfer the grilled chicken to the pot with teriyaki sauce. Cover and place on the grill over medium heat. Steam the chicken in the sauce for 10 minutes to ensure it remains tender, moist, and flavorful.

Remove the chicken from the pot and slice into bite-sized pieces if desired. Arrange on a serving platter and garnish with chopped green onions and toasted sesame seeds. Pair with steamed rice, a green salad, or grilled vegetables.