Add the marinated beef in a single layer and cook for 2 minutes on one side. Flip and cook for another 2 minutes until browned. Do not overcrowd the pan; cook in batches if necessary.
5. Combine Everything
Return the cooked vegetables to the wok.
Pour in the sauce mixture and stir to coat everything evenly.
Add the cornstarch slurry and stir until the sauce thickens.
6. Finish and Serve
Toss in the green onions and stir for another 30 seconds.