Meanwhile, combine soy sauce, stock, mirin and sugar in a small saucepan over medium-high heat. Bring to the boil. Reduce heat to medium. Simmer for 5 minutes or until sauce has reduced and thickened slightly. Allow to cool completely.
Thread chicken and onions onto skewers. Place onto a plate. Brush with sauce.
Preheat grill on medium-high heat until hot. Grill skewers, basting with sauce, for 6 to 8 minutes or until cooked through. Serve with rice.