Soak 10–12 bamboo skewers in water for at least 10–30 minutes.

Separate green and white parts of Tokyo negi. Cut white/light green parts into 1¼-inch pieces. Cut green tops in half crosswise.

In a saucepan, combine soy sauce, mirin, sake, water, brown sugar, and green negi tops. Bring to a boil, then reduce heat and simmer uncovered until reduced to one-third, about 30 minutes. Cool completely; sauce thickens as it cools. Remove green onion before storing.

Tip: Reserve one-third of the sauce separately for final basting.

Cut chicken thighs into 1¼-inch squares.

Fold a piece of chicken and thread onto the skewer. Add a piece of negi, then alternate chicken and negi. Each skewer should have about 3 chicken pieces and 2 negi pieces.

Line a baking sheet with foil and place a greased wire rack on top. Arrange skewers in two rows. Protect skewer handles with foil.

Broil at 550°F (288°C) for 6 minutes. Brush with sauce, flip, and broil 3–4 minutes more. Flip again, baste, and broil 1–2 minutes until caramelized.

Heat neutral oil in a large pan over medium heat. Cook skewers about 5 minutes per side. Cover and cook 2–3 minutes more. Add sauce and cook until coated and glossy.

Transfer to plate presentation-side up. Brush with reserved sauce and sprinkle with shichimi togarashi if desired.