First, combine the marinade ingredients in a bowl. Add the steak strips and toss to coat. Leave in the fridge for up to 24 hrs if you can, or carry on to step 2.
Sprinkle the cornflour over the steak and mix until each piece is coated in a floury paste. Pull the strips apart and arrange over a plate. Drizzle each piece of steak with a little oil. Heat the air fryer to 220C if it has a preheat setting.
Carefully put the beef on the cooking rack in the air fryer, cook for 6 mins, then turn and cook for another 4-6 mins until crispy.
Meanwhile, heat 2 tbsp vegetable oil in a wok over a high heat and stir-fry the garlic, ginger, chilli, pepper and white ends of the spring onions for 2-3 mins until the pepper softens. Be careful not to burn the ginger and garlic. Add the vinegar, soy, sweet chilli sauce and tomato ketchup, mix well and cook for another minute until bubbling.
Tip the beef into the wok and toss through the sauce. Continue cooking for another minute until piping hot, then serve scattered with the spring onion greens and a little extra sauce on the side.
