Slice the Beef: Freeze the steak 20–30 minutes to firm, then slice 1/4-inch thick against the grain. Pat very dry with paper towels.
Marinate the Beef
Combine soy sauce, cornstarch, and sesame oil; marinate for 15 minutes.
Prep Broccoli: Cut into uniform florets. Option 1—Blanch 60–90 seconds in boiling water, drain, and pat dry. Option 2—Microwave with 1 tbsp water for 60–90 seconds, then drain and dry.
Make the Sauce: Whisk remaining soy/tamari with beef broth, brown sugar, ginger, garlic, oyster/hoisin (if using), 1 tsp sesame oil, rice vinegar, and 2 tbsp cornstarch until smooth.
Sear the Beef: Heat the wok over high until smoking. Add 1 tbsp oil, then half the beef in a single layer; sear 45–60 seconds per side. Transfer to a plate; repeat with remaining beef.
Stir-Fry Broccoli: Add 1 tsp oil if needed; stir-fry broccoli 2–3 minutes until crisp-tender with a few charred spots.
Sauce & Combine: Return beef to the wok. Whisk the sauce again and pour in; toss continuously 1–2 minutes until glossy and thickened.
Finish & Serve: Adjust seasoning with more soy, vinegar, or chili. Garnish with scallions and sesame seeds; serve over rice or noodles.