Slice the beef very thinly and marinate for 10 minutes.
Whisk all the sauce ingredients together in a small bowl.
Heat a little oil in a hot wok. Add the beef and let it sear undisturbed for 1 minute, then stir-fry for about 3 minutes more. Transfer to a plate.
Stir-fry the garlic for 1 minute, add the spring onions, and cook for 2 minutes more.
Add the noodles, beef, and sauce. Stir-fry for 3–4 minutes, until the sauce thickens and coats the noodles.
