01 - Combine sliced beef with 1 tablespoon soy sauce and 1 tablespoon cornstarch in a bowl. Mix thoroughly to coat and let marinate for 10 minutes.
02 - Whisk together 2 tablespoons soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, broth, and minced garlic in a small bowl. Set aside.
03 - Mix 1 teaspoon cornstarch with 2 tablespoons water in a separate cup until smooth.
04 - Bring a pot of water to boil. Blanch broccoli florets for 1–2 minutes until bright green and crisp-tender. Drain and immediately rinse with cold water to stop cooking.
05 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add beef in a single layer and sear for 1–2 minutes until browned but not fully cooked. Remove and set aside.
06 - Add remaining oil to the pan. Stir-fry onion for 1 minute, then add blanched broccoli and cook for another 1–2 minutes.
07 - Return beef to the pan. Pour in garlic sauce, stirring to coat all ingredients. Bring to a simmer.
08 - Stir in cornstarch slurry and cook for 1–2 minutes until sauce thickens and beef is cooked through.
09 - Remove from heat. Garnish with sesame seeds and scallions if desired. Serve immediately with steamed rice or noodles.