Cut beef thinly across the grain about ½ inch thick and 2-inches long.
Place beef in a freezer bag or medium bowl and stir in 2 Tablespoons of soy sauce and the 2 Tablespoons cornstarch and allow this to marinate at room temperature for at least 30 minutes.
Next, prepare the vegetables and set aside.
Then make the sauce by whisking together ½ cup soy sauce, brown sugar, garlic powder, and ginger. Set aside.
Heat the oil to high heat in a large deep skillet or wok.
Add half of the marinated beef to the skillet and fry for 3 minutes. Remove and fry the other half of the marinated beef. Remove from the skillet.
Next, add the vegetables (except the green onions) to the skillet and fry for 2 minutes.
Add the beef back to the skillet and stir in the sauce and heat until warmed all the way through.
Sprinkle the green onions and toasted sesame seeds over top.
Serve over noodles or rice if desired.