Dry the chicken fillets with paper towels and cut off any fat. Cut each fillet across at an angle into very thin slices.
In a small bowl, mix together the cornflour, stock and soya sauce. Set aside.
Heat the oil in a wok or non-stick frying pan over high heat until a piece of capsicum sizzles when added to the oil. Add the garlic and ginger and stir-fry for 1–2 minutes or until lightly browned and fragrant.
Add the chicken slices to the wok and stir-fry for 3–4 minutes or until browned and cooked through. Using a slotted spoon, transfer the chicken to a bowl.
Allow the oil in the wok to reheat, then add the sliced vegetables. Stir-fry for about 4 minutes or until softened and lightly browned.
Return the chicken and its juices to the wok and stir in the cornflour mixture. Cook, stirring, until the liquid comes to the boil and thickens slightly. Season with pepper, then serve.