01 - Combine sliced beef with 1 tbsp soy sauce and 1 tbsp cornstarch. Mix thoroughly and let marinate for 10 minutes at room temperature.
02 - Whisk together 2 tbsp soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, broth, and minced garlic until well combined. Set aside.
03 - Mix 1 tsp cornstarch with 2 tbsp water in a small cup until smooth. This will thicken the sauce later.
04 - Bring a pot of water to boil. Add broccoli florets and cook for 1-2 minutes until bright green and crisp-tender. Drain immediately and rinse with cold water to stop cooking.
05 - Heat 1 tbsp vegetable oil in a large wok over high heat. Add beef in a single layer and sear for 1-2 minutes until browned but not fully cooked. Remove beef 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 until heated through.
07 - Return beef to the pan. Pour in garlic sauce and stir to coat all ingredients. Bring to a simmer.
08 - Stir in the cornstarch slurry and cook for 1-2 minutes until sauce thickens and beef is cooked through. The sauce should coat the back of a spoon.
09 - Remove from heat. Sprinkle with toasted sesame seeds and sliced scallions. Serve immediately with steamed rice or noodles.