1.Place noodles in a large heatproof bowl. Pour over boiling water to cover. Set aside to soak for 5 minutes. Rinse under cold water. Drain well.
2.In a jug, combine lime juice, soy sauce, fish sauce and sugar. Set aside.
3.Heat a wok or large frying pan on high. Spray lightly with oil. Add beef in 2 batches. Stir-fry for 2-3 minutes until golden. Transfer to a plate.
4.Return wok to a high heat. Spray with a little more oil. Add lemongrass and chilli. Stir-fry for 1 minute or until fragrant. Toss through choy sum stalks and water. Stir-fry for 1-2 minutes.
5.Add noodles, beef, lime juice mixture and choy sum leaves. Toss together for 2 minutes until heated through. Divide between bowls, top with peanuts. Serve with lime wedges.
Other Asian greens can be used instead of choy sum. If you prefer, replace the beef with chicken breast.Note