Combine oyster sauce, soy sauce and rice syrup in a small jug. Place prawns and mushrooms into separate bowls. Pour 1/4 the soy mixture over 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. Add 1 tbs oil and prawns and stir-fry for 1-2 minutes or until pink. Remove and set aside.
Add remaining oil and onion to wok and stir-fry for 1 minute. Add choy sum stems and asparagus and stir-fry for 1 minute. Add mushrooms and stir-fry for 2 minutes or until mushrooms are almost tender.
Add prawns and choy sum leaves and stir-fry for 1 minute or until leaves just wilt. Serve immediately with rice or noodles.
Note: Prawns can be replaced with sliced chicken, beef or pork fillet.