If using wooden skewers, soak in water for around 30 minutes before using.
Place the soy sauce, mirin and sugar in a small pan and mix. Warm over a medium heat until the sugar dissolves and the mixture comes to a simmer. Reduce by around ⅓ then remove from heat and allow to cool.
Meanwhile, dice the chicken into roughly 1in/ 2 ½cm pieces and cut the white and light green part of the spring onions into 1in/ 2 ½cm lengths.
Preheat the grill to a medium-high heat.
Drain the skewers and thread pieces of spring onion and chicken alternately onto the skewers, trying to keep them so the pieces sit flat.
Place the skewers either directly on the grill or, possibly better, on foil over the grill. Grill for around 4 - 5 minutes until the first side is cooked.
Pour around half of the soy mixture into a small bowl and use this for basting. Reserve the rest for serving. Turn the skewers and baste the top (cooked) side. Cook for a further 3 - 5 minutes until the other side is cooked then turn and baste on top again.
Cook for a further minute or two on the first side before serving with reserved sauce (NOT the sauce used for basting) on the side for dipping.
Video
Notes
This should give you excess sauce so that you have some for dipping. If you prefer, you can just make ⅔ of this amount to be closer to the amount for basting.