Prep the steak: Freeze the steak for 15–20 minutes to firm it up. Slice very thinly against the grain.
Pat dry with paper towels.
Marinate briefly: In a bowl, mix 1 tablespoon soy sauce, 1 teaspoon cornstarch, and 1 teaspoon oil. Toss with the steak. Let sit 10–15 minutes while you prep the veggies and sauce.
Make the sauce: In a small bowl, combine remaining soy sauce, oyster sauce, hoisin (or sugar), rice vinegar, sesame oil, broth, remaining 1 teaspoon cornstarch, and a pinch of red pepper flakes if using.
Stir until smooth.
Heat the pan: Use a large skillet or wok. Heat 1 tablespoon oil over high until shimmering.
Sear the steak in batches: Add half the beef in a single layer. Don’t stir for 45–60 seconds to get a good sear, then stir-fry until just browned, about 1–2 minutes more.
Transfer to a plate. Repeat with 1 tablespoon oil and remaining beef.
Cook the veggies: Add the last tablespoon of oil. Toss in firm veggies first (broccoli, carrots), stir-fry 2 minutes.
Add quicker-cooking veggies (peppers, snap peas, mushrooms, onions) and cook 2–3 minutes more until crisp-tender.
Add aromatics: Push veggies to the sides. Add garlic and ginger to the center. Stir 20–30 seconds until fragrant.
Combine and sauce: Return steak and any juices to the pan.
Stir the sauce, then pour it in. Toss everything for 1–2 minutes until the sauce thickens and coats the beef and veggies.
Taste and adjust: Add a splash more soy for salt, a squeeze of lime for brightness, or chili for heat.
Serve: Spoon over warm rice or toss with cooked noodles. Garnish with green onions and sesame seeds.