Slice the beef into thin strips and toss with soy sauce, olive oil, garlic powder, black pepper, and cornstarch.
Add the sliced bell peppers and onion to the bowl and toss to coat.
Preheat the air fryer to 400°F (200°C).
Arrange the beef and vegetables in a single layer in the air fryer basket.
Air fry for 10-12 minutes, shaking the basket halfway through, until the beef is cooked through and the vegetables are tender.
If desired, toss with a splash of beef broth before serving.