Stir-fry the beef over high heat for 2–3 min, until no longer pink. Set aside.
Add the onion, bell pepper, and celery; stir-fry for 3 min.
1 medium onion, sliced, 1 large green bell pepper, thinly sliced, 2 celery stalks, thinly sliced
Add the beans, the white parts of the bok choy, and the mushrooms; stir-fry for 3 min.
200 g flat green beans, 2 bok choy stalks, thinly sliced (white parts separated from greens), 250 g mushrooms, thinly sliced
Add the green parts of the bok choy; stir-fry for 1 min.
Pour in the sauce and stir until thickened.
Return the beef and toss well for 1 min.
Notes
If you're making a large batch, avoid adding too many vegetables to the wok at once. Otherwise they won't stir-fry properly and will release excess water, leaving you with a boiled dish instead of a stir-fry.