1Thinly 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.
2Heat 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.
3Add 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.
4Add 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.
Tip: Gai Laan can be replaced with other Asian green vegetables like bok choy or choy sum, the same cooking principals should be followed, cook the stems off with the capsicum and add the leaves right at the last minute.