Thinly slice the beef across the grain. Place noodles in a heatproof bowl, cover with boiling water and stand for 2 minutes, drain well, then set aside.
Heat the wok over high heat until hot. Add 2 teaspoons of the oil and swirl to coat the wok. Add one quarter of the beef and stir-fry for 30 seconds until sealed. Remove to a clean bowl. Repeat with oil and beef in three batches.
Add remaining oil, onion and mushrooms to the hot wok, stir-fry for 2 minutes until the mushrooms start to soften. Add gai laan stems and capsicum and stir-fry for 1 minute.
Add noodles and return the beef, add the ketcup manis and sweet chilli sauce and stir-fry until warmed through. Add gai laan leaves, green onions and peanuts. Stir-fry until leaves begin to wilt then remove from the heat. Transfer to bowls and serve.