Place steak in bowl. Season steak with salt and pepper. Toss with cornstarch and stir to coat. Set aside while you prepare the vegetables and sauce.
Prepare sauce ingredients in a medium mixing bowl. Set aside.
In a large saute pan or wok, heat 1/2 tablespoon oil over medium-high heat. When the oil is hot, but not smoking, add the beef and cook until it's browned on all sides, fully cooked through. Most likely, you'll need to separate the beef into 2 small batches so that the beef isn't overcroweded in the pan. Remove the beef and any juices, and sest aside on a plate.
Heat remaining 1/2 tablespoon oil. Add vegetables. Cook approximately 4 minutes until slightly softened and lightly browned.
When the vegetables are ready, add the beef back to the pan. Pour in the sauce and toss to coat. Let simmer 3-4 minutes to thicken the sauce and warm beef through.
Serve warm over rice or noodles Add garnishes such as chopped green onions, sesame seeds, red pepper flakes and/or soy sauce, as desired.