Cut beef thinly across the grain (no more than ½ inch thick, 2 inch long strips). Place it in a freezer bag (or medium bowl). Stir in 2 Tablespoons of soy sauce and the 2 Tablespoons cornstarch and set aside. Allow this to marinate at room temperature for at least 30 minutes.
Prepare the vegetables and set aside.
Prepare the sauce: Whisk together ½ cup soy sauce, brown sugar, garlic powder, and ginger. Set this aside.
Heat the oil to high heat in a large deep skillet or wok.
Add half of the meat to the skillet and fry for 3 minutes. Remove this meat from the skillet and fry the other half. Then remove it from the skillet. (You don’t want to crowd the meat or it will not sear properly.)
Add the vegetables (except the green onions) to the skillet and fry for 2 minutes.
Then add the beef back to the skillet. Stir in the sauce and heat through.
Sprinkle the green onions and toasted sesame seeds over top.
Serve over noodles or rice.