Marinate the beef: Place the sliced beef in a bowl. Add cornstarch, soy sauce, vegetable oil, and black pepper. Toss until evenly coated. Let it rest for 10 minutes.

Make the sauce: In a small bowl, whisk together soy sauce, oyster sauce, honey, beef broth, rice vinegar, sesame oil, garlic, ginger, cornstarch, and water.

Cook the broccoli: Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add the broccoli and onion. Stir-fry for 3–4 minutes until the broccoli is bright green and slightly tender. Remove and set aside.

Cook the beef: Add another tablespoon of oil to the hot pan. Spread the beef into a single layer and cook for 2–3 minutes per side until browned and cooked through.

Combine everything: Return the broccoli and onion to the pan. Pour in the garlic sauce and toss everything together.

Thicken the sauce: Cook for 2–3 minutes, stirring frequently, until the sauce becomes glossy and thick enough to coat the beef and broccoli.

Garnish and serve: Sprinkle with sliced green onions and sesame seeds. Serve hot over steamed white rice, brown rice, or gluten-free noodles.