In a heavy-bottomed pot, add water, shoyu, brown sugar, garlic, ginger and green onion whites, and whisk to combine. Nestle the chicken thighs in the pot, submerging them in the shoyu braising liquid as much as possible.
Bring to a boil over high heat, then reduce to low. Cover the pot and simmer for 30-35 minutes, until the chicken is tender and cooked through.
(Optional step) Preheat oven broiler to 500 degrees. Make sure oven rack is on the top shelf. Transfer the chicken thighs to a small baking sheet. Place in the oven for 4-5 minutes, rotating the baking sheet halfway through, until the chicken skin is crispy and caramelized. As the chicken broils, bring the sauce in the pot to a boil by increasing the heat to high for 4- 5 minutes or until sauce is reduced to desired thickness.
Spoon the thickened shoyu sauce over the chicken and garnish with thinly sliced green onion.
Serve Shoyu Chicken immediately alongside rice and mac salad.