Prepare the Sauce:
In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, rice vinegar, garlic, ginger, and crushed red pepper flakes (if using). Set aside.
Cook the Beef:
Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat.
Add the beef slices in a single layer and cook for 2-3 minutes on each side, until browned and cooked through. Remove the beef from the pan and set aside.
Sauté the Vegetables:
In the same skillet, add the remaining tablespoon of oil.
Add the broccoli, bell pepper, carrot, and onion. Sauté for 4-5 minutes, or until the vegetables are tender but still crisp.
Combine and Stir-Fry:
Return the beef to the skillet with the vegetables. Pour the stir-fry sauce over the beef and vegetables. Stir to coat evenly.
Cook for an additional 2-3 minutes, allowing the sauce to thicken slightly and everything to heat through.
Finish and Serve:
Drizzle the sesame oil over the stir-fry and toss to combine.
Serve the beef and broccoli stir-fry over cooked rice and enjoy!