Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add minced garlic and stir-fry for about 30 seconds until fragrant.
Add sliced flank steak in a single layer. Cook for 2-3 minutes until browned.
Push the beef to one side of the skillet. Add another tablespoon of oil if needed, then add broccoli florets and sliced bell pepper.
Stir-fry the vegetables for 3-4 minutes until they are tender-crisp.
Combine beef with vegetables in the skillet.
In a small bowl, whisk together soy sauce, oyster sauce, brown sugar, sesame oil, salt, and pepper.
Pour the sauce over the beef and vegetables. Stir-fry for another 1-2 minutes until everything is coated and heated through.
Serve hot over cooked rice.