Combine mirin, soy sauce, sugar, and pressed garlic in a medium saucepan. Simmer over medium heat until the mixture reduces by half, then remove from heat and let cool slightly to thicken into a brushable glaze.

Cut chicken thighs into bite-sized pieces, keeping the skin on. Thread the chicken pieces onto pre-soaked bamboo skewers, alternating with pieces of green onion.

Grill the skewers over medium heat. Start by cooking without sauce to sear the chicken. Once it begins to change color, brush with the tare sauce. Turn and brush the skewers approximately three times total, allowing the sauce to caramelize in thin layers for a glossy, charred finish.