1Mix the marinade ingredients (soy sauce, sesame oil, powdered sugar, and five-spice powder) in a bowl. Add the beef strips and marinate, ideally up to 24 hours.
2Sprinkle cornstarch over the meat and mix well. Drizzle with a little vegetable oil. Preheat the air fryer to 200°C (400°F).
3Place the meat in the air fryer basket and cook for 6 minutes, then turn and cook another 4-6 minutes until crispy.
4Meanwhile, heat 2 tablespoons of vegetable oil in a wok or large pan. Stir-fry garlic, sliced ginger, chili pepper, bell pepper strips, and white parts of green onions for 2-3 minutes.
5Add rice vinegar, soy sauce, sweet chili sauce, and ketchup. Stir and cook for 1 more minute.
6Pour the crispy beef from the air fryer into the wok and mix well with the sauce. Cook for 1 more minute, then serve immediately, sprinkled with the green parts of the onions.