Combine the mirin, soy sauce, chicken stock, Colman's mustard, and garlic in a saucepan. Simmer on a low heat for 30 minutes until the sauce starts to thicken.
While the sauce thickens, soak your skewers in water and get your roasties on.
Slice the chicken into strips, place on the skewers with spring onion, and brush with lots of mustard. Season before barbecuing with the saucy glaze. Enjoy!