Soak bamboo skewers in water for 30 minutes. Cut the green tops from 2 Tokyo negi (long green onions).
For the sauce, bring ½ cup of soy sauce, ½ cup of mirin, ¼ cup of sake, ¼ cup of water, 2 teaspoons of brown sugar, and the negi tops to a boil in a small saucepan.
Simmer for about 30 minutes, until reduced to a third, then lift out the negi tops.
Cut 450g of boneless, skinless chicken thighs and the white parts of the negi into 1¼in / 3cm pieces. Thread them alternately onto the skewers, about 3 pieces of chicken and 2 of negi on each.
Heat the broiler. Broil the skewers on a baking sheet for 6 minutes, then brush with the sauce and broil for 3-4 minutes.
Turn the skewers, brush with more sauce, and broil for 1-2 minutes, until glazed and the chicken reaches 165°F / 74°C.
