Slice the flank steak against the grain into 1/4-inch thin strips. Toss lightly with 1 tablespoon of cornstarch to ensure a crispy sear. In a small bowl, whisk together low-sodium soy sauce, water, brown sugar, rice vinegar, minced garlic, and grated ginger until blended.

Heat 1 tablespoon of toasted sesame oil in a large wok or cast-iron skillet over high heat. Add the broccoli florets and 2 tablespoons of water; cover for 2 minutes to steam until bright emerald green and crisp-tender. Remove broccoli and set aside.

Add the remaining 1 tablespoon of sesame oil to the smoking wok. Add beef strips in a single layer without overcrowding. Flash-sear undisturbed for 90 seconds until a deep, caramelized crust forms, then flip and sear 1 minute more.

Return the steamed broccoli florets to the skillet. Pour the prepared teriyaki sauce over the beef and broccoli. Stir in the cornstarch slurry and simmer for 60-90 seconds until the sauce bubbles vigorously and transforms into a thick, glossy lacquer coating every strip.

Remove from heat immediately. Garnish generously with toasted sesame seeds and sliced green onions. Serve piping hot over fluffy jasmine rice.