In a small saucepan, combine ½ cup soy sauce, ¼ cup mirin, ¼ cup sake, 2 tablespoons brown sugar, 1 minced garlic clove, and 1 teaspoon grated ginger. Bring to a simmer over medium heat and cook for 5 minutes until slightly thickened. Set aside.
Thread 1½ pounds chicken thigh pieces and 3 chopped green onions onto soaked skewers, alternating chicken and green onion.
Preheat a grill pan or outdoor grill over medium high heat.
Place the skewers on the grill and cook for 2 to 3 minutes, then brush with some of the prepared sauce.
Turn the skewers and cook for another 2 to 3 minutes, then brush again with the sauce.
Continue cooking and turning the skewers for a total of 8 to 10 minutes, brushing with sauce each time, until the chicken is fully cooked and slightly caramelized.
Remove from grill and brush one last time with the sauce before serving.