In a small saucepan, combine soy sauce, mirin, sake, water, brown sugar, garlic, ginger, and the green onion top. Bring to a boil over medium-high heat, then reduce heat and simmer for 15 to 20 minutes until the sauce reduces by half and becomes glossy and thick. Set aside to cool. Reserve about 2 tablespoons of the tare in a separate bowl for final drizzling.

Thread the chicken and green onion pieces onto soaked bamboo skewers, alternating between them. Use 3 to 4 pieces of chicken and 2 to 3 pieces of onion per skewer, leaving small spaces between pieces for even cooking.

Preheat your grill, grill pan, or broiler to medium-high heat. Lightly oil the grates with a paper towel dipped in vegetable oil.

Place the skewers on the grill and cook for 2 to 3 minutes on one side until lightly browned with char marks.

Brush the skewers generously with the tare, flip them over, and cook for another 2 to 3 minutes.

Continue flipping and brushing the chicken with tare every minute until the chicken is cooked through (internal temperature of 165 degrees F) and the sauce has caramelized into a dark, sticky glaze, about 8 to 10 minutes total.

Transfer skewers to a serving plate, drizzle with the reserved fresh tare, and serve immediately.