Marinate beef for 15 minutes. Mix sauce ingredients in a bowl. Separate fresh noodles or prepare dried rice noodles according to instructions.
Heat pan over high heat. Sear beef in batches for 2–3 minutes until browned and slightly charred, letting it sit undisturbed for 1 minute before flipping. Remove from pan.
Add oil, then onion, garlic and ginger. Stir-fry for 1–2 minutes until fragrant. Add Chinese broccoli stems and cook for 2 minutes, then add leaves and cook a further 1–2 minutes until wilted. Remove from pan.
Add noodles to the hot pan and let sit undisturbed for 30 seconds to lightly char. Toss gently, being careful not to break the noodles, then add sauce and cook for 1 minute until evenly coated.
Return beef and vegetables to the pan and toss for 1–2 minutes until glossy and combined. Add bean sprouts and spring onions in the final 30 seconds. Serve immediately with chilli oil and chilli flakes if desired.
