Dry the chicken fillets. 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 non-stick frying pan over high heat until a piece of pepper 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 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 pan to reheat, then add the sliced vegetables and mushrooms. Stir-fry for about 4 minutes or until softened and lightly browned.
Return the chicken and its juices to the pan and stir in the cornflour mixture. Cook, stirring, until the liquid comes to the boil and thickens slightly. Season with the pepper, then serve.