1. Soak 10 wooden skewers for 15 minutes in water.
2. Cut the chicken into 2.5cm cubes. Thread the chicken cubes onto the soaked skewers, alternating with the spring onion.
3. In a small saucepan, combine the mirin, soy sauce, sake and sugar. Bring to the boil. Reduce heat and simmer for 5 minutes. Reserve half of the sauce for dipping.
4. Heat a grill pan over high heat and lightly brush with vegetable oil. Cook the skewers, 7 to 10 minutes per side, basting frequently with half of the sauce. Remember to baste frequently for best flavor!
5. Serve with extra sauce on the side, and with white rice if desired.