Get the rice going first — it cooks while you stir-fry. Slice the flank steak thinly against the grain into 1/8-inch strips (partially freezing it for 15 minutes makes this much easier). Toss the strips with 1 tablespoon soy sauce and 1 tablespoon cornstarch; set aside while you prep everything else.

In a small bowl, whisk the remaining 1/3 cup soy sauce, oyster sauce, honey, rice vinegar, sesame oil, 1 tablespoon cornstarch, and the water until smooth. Cut the broccoli into bite-size florets and mince the garlic and ginger.

Heat 2 tablespoons oil in a wok or large skillet over high heat until shimmering and just starting to smoke. Sear the beef in a single layer in two batches, 1–2 minutes per side, until browned at the edges and cooked through. Transfer to a plate — don't crowd the pan or the beef steams instead of sears.

Add the remaining 1 tablespoon oil to the same pan. Add the broccoli and 2 tablespoons of water, cover, and steam 2 minutes. Uncover and cook 2–3 minutes more, stirring occasionally, until bright green with charred edges and crisp-tender bite.

Push the broccoli to the sides. Add the garlic, ginger, and red pepper flakes to the center; cook 30 seconds until fragrant. Return the beef to the pan, pour in the sauce, and toss everything 1–2 minutes until the sauce turns glossy and thick enough to coat a spoon.

Spoon over hot jasmine rice, shower with sesame seeds and green onions, and serve immediately.