Combine beef with the marinade ingredients. Set aside.
Combine the stir-fry sauce ingredients and set aside.
Heat oil in a wok or large pan over high heat. Add the beef and allow to sear before stir-frying. Add the garlic, onion and ginger, and stir-fry for one minute. Then add mushrooms and stir-fry for 2 minutes.
Pour over the stir-fry sauce and toss to combine.
Toss through some scallions/spring onions. Transfer to a serving plate.