In a large bowl, toss the thinly sliced steak with cornstarch until evenly coated.
Place the coated steak in a single layer in the air fryer basket. Spray lightly with cooking oil. You may need to cook in batches to avoid overcrowding.
Air fry the steak at 400°F (200°C) for 8-10 minutes, flipping the steak halfway through. Remove and set aside.
In a saucepan, heat the vegetable oil over medium heat. Add the garlic and ginger, and cook for about 1 minute until fragrant.
Pour the soy sauce, water, brown sugar, hoisin, and rice vinegar into the pan and whisk to combine. Bring to a simmer. Cook for 10-12 minutes, stirring occasionally, until the sauce thickens slightly. If desired, add red pepper flakes for a spicy kick.
Add the steak to the pan with the sauce and toss to coat. Stir until all the beef is coated and the sauce thickens, about 3 minutes.
Transfer to a serving dish and garnish with green onions and sesame seeds.
Serve over steamed rice.
Notes
The sauce will thicken more when you add the beef.
All air fryers have different wattage and may cook at different rates. You may need to adjust the time or temp for the best results.