Marinate the beef. Combine vinegar, soy sauce, honey, 1 tablespoon ginger, red pepper flakes, and cumin. Toss the steak to coat and let sit for 15 minutes.
Make the slurry. Mix cornstarch and water in a small bowl — this will thicken the sauce later.
Sear the beef. Heat vegetable and sesame oil in a wok or skillet over high heat. Pat the steak dry and sear both sides until browned, about 30 seconds per side. Remove and set aside.
Build flavor. Add garlic, serranos, and the remaining ginger to the pan. Stir until fragrant, about 30 seconds.
Finish the stir‑fry. Return the steak to the pan with green onions and the slurry. Stir for about a minute until everything’s glossy and coated.
Serve. Top with chopped cilantro and serve immediately.