Preheat your oven to 375°F.
In a large bowl, whisk together soy sauce, water, sugar, mirin, garlic, and ginger until the sugar dissolves completely.
Heat vegetable oil in a large oven-safe skillet over medium-high heat, then brown the chicken thighs skin-side down for 4-5 minutes until golden and crispy.
Flip the chicken pieces, then pour the sauce mixture over the chicken in the skillet.
Transfer the skillet to the preheated oven and bake for 35-40 minutes, basting the chicken with the sauce every 15 minutes, until the chicken is cooked through and the sauce has thickened into a glaze.
Remove from oven, let rest for 5 minutes, garnish with sliced green onions, and serve over steamed white rice.