Mix the cornflour with the soy sauce and lemon juice in a small bowl.
Heat the oil in a large frying pan or wok over a high heat, add the chicken, capsicum, carrot and broccoli and stir-fry for 2–3 minutes, or until the chicken is lightly browned and the vegetables are beginning to soften.
Pour the lemon and soy mixture into the pan, add the chicken stock and spring onions and bring to a simmer. Reduce the heat and cook for 2 minutes, or until the sauce is slightly thickened and the chicken is cooked through, stirring regularly.
Sprinkle with grated lemon zest and serve with rice.