1.Whisk stock, sauce, cornflour, sugar and sesame oil in small bowl until well combined.
2.Heat half of the vegetable oil in wok. In batches, stir-fry chicken until just cooked through.
3.Heat remaining vegetable oil in wok. Stir-fry onion, ginger, garlic, broccoli and water until broccoli is tender.
4.Return chicken to wok with sauce mixture; stir-fry until chicken is hot and sauce boils and thickens slightly.