The cornflour is the whole point. Without it the sauce pools sadly in the bottom of the wok; with it, it clings to every single piece of chicken and veg like a takeaway.
why it works
Zero saturated fat and no hidden industrial oils, which is more than can be said for any bottled stir-fry sauce. Making it yourself also means the soy quantity is a decision rather than an accident.
Whisk everything together in a small cup before you turn the hob on. Slake the cornflour in the cold water separately first — dump it in dry and you'll get lumps you can't rescue.
Fry the chicken until golden and cooked through, then remove it to a plate.
Crank the heat and flash-fry the veg for 2–3 minutes so it stays bright and crunchy.
Chicken back in with the veg. Give the sauce a quick stir, because the cornflour will have settled, and pour it straight in.
Bubble for just 60 seconds. The cornflour activates in the heat and turns the watery liquid into a thick, glossy glaze.