In a bowl, toss the thinly sliced beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, and 1 teaspoon sesame oil. Let rest for 10 minutes while you prep the broccoli.

Bring a pot of salted water to a boil and add the broccoli florets for 1 minute until bright green and slightly tender.

Drain and plunge into ice water to stop cooking; then drain again.

In a small bowl, combine 2 tablespoons soy sauce, oyster sauce (if using), rice vinegar or sherry, brown sugar, 1/2 cup beef broth, grated ginger, and minced garlic. Stir until sugar dissolves.

Heat a large wok or skillet over high heat. Add 1 tablespoon vegetable oil and swirl.

Add half the beef in a single layer and sear for about 30–45 seconds per side until browned but not overcooked. Remove and repeat with the remaining beef.

Lower heat to medium-high, add the remaining oil, then pour in the prepared sauce mixture. Bring to a simmer, scraping up any browned bits from the pan.

Mix 1 teaspoon cornstarch with 1 tablespoon water and whisk into the simmering sauce. Cook for 30 seconds until glossy and slightly thickened.

Return the beef and broccoli to the pan and toss to coat, heating through for 1–2 minutes.

Drizzle the remaining teaspoon of sesame oil over the dish, add a pinch of red pepper flakes if you like warmth, and serve immediately over steamed rice.