Prep the beef: Freeze the steak for 15–20 minutes to firm it up. Slice very thinly against the grain. Pat dry and season lightly with salt and pepper.

Make the teriyaki sauce: In a bowl, whisk soy sauce, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes (if using). In a separate small cup, mix cornstarch with water to make a slurry and set aside.

Blanch or steam broccoli (optional, for extra-crisp results): Steam florets for 2–3 minutes until bright green and just tender, then drain well. If skipping this step, plan to cook broccoli a bit longer in the skillet.

Heat the skillet: Set a large skillet or wok over medium-high heat. Add 1 tablespoon oil and swirl to coat.

Sear the beef in batches: Add half the beef in a single layer. Sear 1–2 minutes per side until just browned. Transfer to a plate. Repeat with remaining oil and beef. Do not overcook.

Sauté aromatics: Reduce heat to medium. Add a splash more oil if the pan looks dry. Add garlic and ginger (if you reserved some) and cook 20–30 seconds until fragrant, scraping up browned bits.

Cook the broccoli: Add broccoli to the pan. Stir-fry 2–3 minutes (longer if not pre-steamed) until crisp-tender.

Add sauce and thicken: Pour in the teriyaki mixture and bring to a simmer. Stir in the cornstarch slurry and cook 30–60 seconds until the sauce turns glossy and thickens slightly.

Return beef and finish: Add the beef and any juices back to the pan. Toss to coat. Drizzle with sesame oil, taste, and adjust with a splash of soy or a pinch of sugar if needed.

Serve: Top with green onions and sesame seeds. Serve hot over rice or noodles.