Place flank steak in the freezer for 15 minutes until firm, then slice thinly against the grain into 1/8 to 1/4 inch thick strips.
In a medium bowl, combine sliced beef with 2 tablespoons soy sauce, 2 teaspoons cornstarch, and baking soda if using. Toss well and let marinate for at least 15 minutes at room temperature.
Cut broccoli into small, bite-sized florets and toss with olive oil in a separate bowl until lightly coated.
Preheat air fryer to 400°F (200°C) for about 3 minutes.
Shake off excess marinade and arrange beef strips in a single layer in the air fryer basket. Air fry at 400°F (200°C) for 6 minutes, shaking the basket halfway through.
Add oiled broccoli florets to the air fryer basket on top of the beef. Air fry for 5-6 more minutes at 400°F (200°C), shaking once or twice, until broccoli is crisp-tender and slightly charred.
In a small saucepan, whisk together remaining soy sauce, oyster sauce, brown sugar, beef broth, sesame oil, garlic, and ginger. Bring to a gentle simmer over medium heat.
Stir in cornstarch slurry and cook for 1-2 minutes, stirring constantly, until sauce thickens enough to coat the back of a spoon.
Pour thickened sauce over beef and broccoli in the air fryer basket or transfer to a large bowl. Toss gently to coat everything evenly. Garnish with sesame seeds and sliced green onions and serve immediately.