Combine cornstarch and water in a glass, mix well, set aside.
Combine all remaining sauce ingredients in a bowl, mix well, set aside.
Heat a wok or large frying pan (I actually use a stand-alone electric frying pan) to medium-hot. Season chicken pieces with salt and pepper.
Add cooking oil to pan and stir-fry chicken for about 5 minutes or until browned.
Add vegetables to pan and continue cooking for about 2-3 minutes (until slightly soft and if frozen heated through).
Add sauce mixture (not corn starch) to chicken and vegetables and bring to a boil. Reduce to medium heat and let simmer for about 5 minutes until chicken is cooked through (the inside of the biggest piece should be at least 160C if you're using a meat thermometer).
Stir in cornstarch mixture and bring to a boil for about one minute (until thickened).