In a large bowl, toss the thinly sliced steak with cornstarch until evenly coated.
Heat about 1/4 cup of cooking oil in a large wok or skillet. When hot, add the beef in a single layer. You may need to cook in batches to avoid overcrowding.
Cook the steak for 2 minutes per side, without touching. When crispy and golden, transfer to a plate and set aside.
To cook in the air fryer: Spread the beef out in a single layer, if possible. Spray with cooking oil. Air fry the steak at 400°F (200°C) for 8-10 minutes, flipping halfway through. Remove and set aside.
In the same pan, over medium low heat, add the garlic and ginger, and cook for about 30 seconds, until fragrant.
Pour in the soy sauce, water, brown sugar, hoisin, rice vinegar, and red pepper flakes. Whisk to deglaze the bottom of the pan and combine the sauce ingredients. Simmer for about 10 minutes, stirring occasionally, until the sauce thickens slightly.
Add the beef back into the pan with the sauce and toss to coat. Stir until all the beef is coated and the sauce thickens, about 3 minutes.
Cook the rice in a rice cooker, on the stovetop or in the Instant Pot, according to package instructions.
Heat a pot of water to boiling over high heat, then turn down to maintain a simmer. Insert a steamer basket and set the broccoli in the basket. Cover and steam for 5 minutes (less if you like it crunchy, more if you like it soft).
Serve the Mongolian beef over steamed rice with broccoli, garnished with green onions and sesame seeds.