Make the marinade by combining all the ingredients for the marinade together in a bowl. Bring to a boil and reduce to a gentle simmer. Cook until the sauce reduces and becomes glossy. This will take around about 10-15 minutes.
If the sauce isn’t thick enough, bring it to a boil and stir in the cornflour slurry.
Allow to cool completely then mix half the marinade with the chicken and set aside for at least 20 minutes. Reserve the remaining marinade to baste the chicken later.
Heat an overhead grill to high. Once hot, place the chicken on a foil lined grill tray and grill for 10-12 minutes on either side until cooked through. Baste the chicken with the pan juices and brush with fresh teriyaki sauce every 5 minutes.
Slice, and garnish with spring onion to serve.