Slice the beef: Freeze the steak for 20 minutes to firm it up.
Slice very thinly against the grain for tender bites. Pat dry with paper towels to help browning.
Mix the sauce: In a bowl, whisk soy sauce, beef stock, oyster sauce, rice vinegar, honey, cornstarch, black pepper, and red pepper flakes. Stir until smooth with no clumps.
Marinate briefly: Add half the sauce to the sliced beef.
Toss to coat and let it sit for 10–15 minutes while you prep the broccoli. Reserve the other half of the sauce for cooking.
Prep the broccoli: Cut into small florets for quick, even cooking. Rinse and shake off excess water.
Quick-blanch (optional but ideal): Bring a pot of salted water to a boil.
Cook broccoli for 1–1.5 minutes, then drain and rinse with cold water. This keeps it bright green and crisp-tender.
Heat the pan: Use a large wok or skillet over medium-high heat. Add 1 tablespoon oil and the white parts of the green onions.
Stir-fry 30 seconds until fragrant.
Sear the beef: Add half the beef in a single layer. Cook 1–2 minutes per side until browned but not fully cooked through. Transfer to a plate and repeat with remaining beef, adding more oil if needed.
Build flavor: Lower heat slightly.
Add garlic and ginger to the pan with a small splash of oil. Stir for 30 seconds, scraping up brown bits.
Add broccoli and sauce: Return broccoli to the pan. Pour in the reserved sauce and bring to a simmer.
Stir for 1–2 minutes as it thickens and glazes the broccoli.
Finish the dish: Return beef (and any juices) to the pan. Toss everything together for 1–2 minutes until the beef is just cooked through and glossy. Drizzle with sesame oil and toss again.
Taste and adjust: Add a splash more soy for salt or a pinch of sweetener for balance.
Top with the green onion tops and sesame seeds if using.
Serve: Spoon over rice, quinoa, or cauliflower rice, or enjoy as a high-protein bowl on its own.