In a large mixing bowl, combine soy sauce, water, and sugar. Stir until the sugar dissolves. Add the mashed garlic and thinly sliced ginger to the marinade.

Place the chicken thighs into the marinade, ensuring they are fully submerged and coated. Cover with plastic wrap or use a resealable plastic bag. Refrigerate for at least 24 hours, or up to 3 days for maximum flavor absorption.

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

Remove the chicken from the marinade, letting excess liquid drip off. Grill the chicken thighs for 5-7 minutes per side, or until fully cooked and charred. Ensure the internal temperature reaches 165°F (75°C).

In a pot or grill-safe pan, add a small amount of fresh teriyaki sauce. This will be used for steaming the chicken after grilling.

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.