Marinade
Sauce
Marinate the beef: stir together marinade ingredients in a medium bowl. Add beef slices and stir until coated. Let stand for 10 minutes.
Prepare the sauce: stir together sauce ingredients in a small bowl.
Blanch the broccoli: Cook the broccoli in a small pot of boiling, salted water until tender-crisp, about 2 minutes. Drain thoroughly.
Heat a large frying pan or wok over high heat until a bead of water sizzles and instantly evaporates on contact. Add cooking oil and swirl to coat. Add beed and immediately spread beef out over all the surface of the pan in a single layer. Let beef fry undisturbed for 1 minute. Flip beef slices over, add garlic and fry for an additional 30 seconds to 1 minute until no longer pink.
Pour in the sauce, add the blanched broccoli and bring to a boil. Pour in the dissolved cornstarch and cook, cstirring, until the sauce boils and thickens, about 30 seconds.