Beat the eggs with the sesame oil and coriander, if using, then set aside. Mix the cornflour to a smooth paste with the soy sauce and sherry, then set aside.
Heat half the vegetable oil in a large frying pan or wok. Add the broccoli, capsicum, ginger, garlic and spring onions. Stir-fry for 5 minutes or until the broccoli looks bright green and is just tender. Pour in the cornflour mixture and stir for 1 minute.
Add the prawns and cook for 2 minutes. Add the bean sprouts and cook for a further 1 minute or until the prawns are pink. Transfer to a large bowl, including all the juices and keep warm. Wipe the pan with a piece of paper towel.
Add the remaining vegetable oil to the pan and heat it for a few seconds. Pour in just over half the egg mixture and swirl the pan to spread it out evenly. Stir once or twice then let the egg set for about 3 minutes. Loosen the omelette with a spatula and transfer it to a serving dish.
Add the remaining egg mixture to the pan and cook for 1–2 minutes until just set. Turn out onto a board and cut into ribbons. Spoon the vegetables on top of the whole omelette then scatter with the omelette ribbons to serve.