Begin by cutting boneless skinless chicken thighs into 1-inch pieces to ensure even cooking.

In a medium bowl, whisk together soy sauce, mirin, sake, brown sugar, freshly grated ginger, minced garlic, and black pepper until brown sugar dissolves.

Add the chicken pieces to the marinade, ensuring they are well-coated. Cover and refrigerate for at least 30 minutes.

Preheat your grill to medium-high heat, around 400°F.

Take the soaked wooden skewers and thread the marinated chicken pieces onto them, alternating with green onions if desired.

Place the skewers on the preheated grill and cook for 5-7 minutes per side, brushing with leftover marinade.

Once cooked, remove skewers and let rest for a few minutes before serving with garnished green onions.