Make the egg mixture: in a bowl, crack the eggs and whisk lightly. Add the light soy sauce, sesame oil and cornstarch slurry, and whisk lightly to combine.
To make the gravy or sauce: in a pan over low heat, combine the chicken stock, light soy sauce, cornstarch slurry, and sesame oil, and leave to simmer on low to reduce to a thick sauce while you're making the eggs.
To the egg mixture: add the bean sprouts, the white ends of the green onions, and char siu pork and combine well.
To the wok: add the oil and heat up to 190°C (375°F), using a kitchen thermometer to check the temperature. Carefully add a small ladle full of the egg mixture to the wok, making sure to get a little of each ingredient in the mix.
The egg mixture should bubble up, form into an omelette, and start to brown on the bottom in around 30 seconds. When it does, use a fish slice to carefully flip the egg over. If it's ready, it should turn over easily with the fish slice.
Cook the other side of the omelette for around 30 seconds, then remove and drain the omelette on kitchen towels. Note: if you're cooking for a crowd, keep the finished omelettes in an oven over low-medium heat.
Serve the omelettes with steamed rice, spoon the reduced gravy over the omelettes, and sprinkle some bean sprouts, white sesame seeds and green onion slices onto each omelette.