Toss the sliced flank steak with the baking soda, 1 tbsp soy sauce, and 1/2 tbsp cornstarch. Rest for 15 minutes at room temperature.

Whisk together the remaining 2 tbsp soy sauce, oyster sauce, brown sugar, sesame oil, and beef broth in a small bowl. Mix the 2 tsp cornstarch with 2 tbsp water in a separate cup to make the slurry.

Heat 1 tbsp vegetable oil in a wok over high heat. Add the broccoli florets and 2 tbsp water, cover, and steam for 2 minutes until bright green and just tender. Transfer to a plate.

Add the remaining 1 tbsp vegetable oil to the hot wok. Sear the marinated beef in a single layer for 90 seconds, flip, and cook another 60 seconds. Cook in batches if needed.

Push the beef aside, add the garlic and ginger, and stir for 30 seconds. Pour in the sauce, bring to a simmer, then stir in the slurry. Cook for 60 to 90 seconds until the sauce is glossy and thick.

Return the broccoli to the wok, toss to coat, then spoon over steamed rice and drizzle with sesame oil and a pinch of sesame seeds to serve.