Place flank steak in freezer for 15-20 minutes to firm up, then slice against the grain into thin strips about 1/4-inch thick. In a bowl, combine beef with water and baking soda, massage until absorbed, then add cornstarch, soy sauce, and oil; marinate for 15 minutes.
In a small bowl, whisk together soy sauce, oyster sauce, brown sugar, broth, dark soy sauce, cornstarch, sesame oil, and white pepper until smooth with no lumps. Set aside.
Bring a large pot of salted water to boil, add broccoli florets and blanch for 30-60 seconds. Immediately transfer to ice water to stop cooking, then drain thoroughly and set aside.
Heat wok or skillet over high heat until smoking, add 2 tablespoons oil, then add beef in single layer. Sear undisturbed for 60-90 seconds, flip and cook another 60 seconds until 80% cooked, then remove to a plate.
Reduce heat to medium-high, add remaining 1 tablespoon oil to pan, then add garlic and ginger. Stir-fry for 15-30 seconds until fragrant, being careful not to burn.
Add blanched broccoli and toss for 30 seconds, then return beef with juices to pan. Stir sauce to recombine, pour over beef and broccoli, and toss for 1-2 minutes until sauce thickens and becomes glossy.
Transfer to serving platter, garnish with sesame seeds and green onions if desired. Serve immediately over steamed rice or noodles.