Combine oyster sauce, soy sauce and brown sugar in a small jug. Place prawns and mushrooms into separate bowls. Pour quarter of the soy mixture over the prawns and remaining soy mixture over mushrooms. Toss well to coat. Cover and place in the fridge to marinate for 15 minutes.

Heat a wok over high heat until hot. Spray with cooking spray and stir-fry the prawns for one to two minutes or until pink. Remove and set aside.

Add onion to wok and stir-fry for one minute. Add choy sum stems and asparagus and stir-fry for one minute. Add mushrooms and stir-fry for two minutes or until mushrooms are almost tender.

Add prawns and choy sum leaves and stir-fry for one minute or until leaves just wilt.

Serve immediately.