In a small bowl, whisk together beef broth, soy sauce, oyster sauce, rice vinegar, honey, chili paste, and cornstarch until smooth. Set aside.
Season sliced flank steak with salt and pepper.
Heat oil in a wok or large skillet over high heat until smoking.
Add beef in a single layer and sear for 2-3 minutes until browned. Cook in batches if necessary to avoid overcrowding. Remove beef and set aside.
In the same wok, stir-fry garlic and onion for 30 seconds until fragrant.
Add bell peppers and broccoli (if using), stir-fry for 2-3 minutes until vegetables are tender-crisp.
Return beef to the wok and pour in the sauce mixture.
Cook for 4-5 minutes, stirring constantly, until sauce thickens and coats the beef and vegetables.
Serve immediately over steamed rice.