1. While beef is still partially frozen, trim away excess fat and slice thinly.
2. Whisk together marinade and pour over beef in a ziplock bag; refrigerate several hours or overnight.
3. Whisk together sauce in a small bowl and keep nearby.
4. Heat oil in a wok over high heat, stir-fry beef with marinade, mushrooms and onions until cooked through.
5. Add sauce to wok, and cook, stirring, until sauce thickens.
6. Serve stir-fry with steamed rice if desired.