In a bowl, whisk together honey, soy sauce, rice vinegar, cornstarch, garlic, ginger, red pepper flakes (optional), and black pepper until smooth. Add the sliced beef and let it marinate for at least 30 minutes.
Heat vegetable oil in a wok over medium-high heat. Cook the marinated beef in batches for 2-3 minutes until browned and cooked through. Set aside.
In the same wok, add more oil if necessary and stir-fry broccoli and bell pepper for about 3-5 minutes until tender-crisp.
Return the beef to the wok and combine with vegetables. Stir-fry for another minute until heated through.
Serve over steamed rice or noodles garnished with sesame seeds and green onions.