01 - In a small saucepan, combine the soy sauce, mirin, sake, sugar, garlic, and ginger. Bring to a gentle boil over medium heat, stirring until the sugar fully dissolves. Reduce the heat and simmer for 5 to 7 minutes until the sauce slightly thickens. Remove from heat and let cool.
02 - Thread the chicken thigh pieces and spring onion pieces alternately onto the soaked bamboo skewers, distributing them evenly.
03 - Brush a grill or griddle pan with vegetable oil and preheat over medium-high heat until the surface is hot and lightly smoking.
04 - Arrange the skewers on the hot grill. Cook for 3 to 4 minutes on each side, brushing generously with the tare sauce as they cook, until the chicken is cooked through and develops a glossy, caramelized glaze.
05 - Transfer the skewers to a serving plate and serve hot, with additional tare sauce on the side for dipping if desired.