Heat a small saucepan and dry-fry the sesame seeds, shaking the pan frequently, until they are just beginning to brown. Turn the seeds out into a small bowl and set aside.
Heat the oil in a wok or large frying pan. Add the onion and garlic and stir-fry for 2–3 minutes or until softened slightly. Add the cabbage and Chinese leaves and stir-fry over a medium–high heat for 2–3 minutes or until the vegetables are just beginning to soften. Add the bean sprouts and cook for a few seconds.
Make a space in the centre of the pan. Pour in the oyster sauce and water, stir until hot, then toss the vegetables into the sauce. Season with pepper to taste. Serve immediately, sprinkled with the toasted sesame seeds.