Cut the steak thinly across the grain. Make sure that steak is cut into 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 2 Tablespoons cornstarch, then set aside to marinate at room temperature for at least 30 minutes.
Prepare the sauce by whisking together ½ cup soy sauce, brown sugar, garlic powder, and ginger. Set aside. Prepare vegetables by cleaning and chopping.
Heat 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 from skillet and fry the remaining beef. You don’t want to crowd the meat or it will not sear properly. Once cooked, remove from skillet and set aside.
Add the vegetables, minus the green onions, to the skillet and stir fry for 2 minutes.
Add the beef back to the skillet.
Stir in the sauce and heat thoroughly, usually 1-3 minutes. Sprinkle the green onions and toasted sesame seeds over top, then serve over rice, noodles or potatoes.