In a bowl, toss the sliced steak with cornstarch until lightly coated.
In a small bowl, whisk together the soy sauce, oyster sauce, sesame oil, and beef broth. Set aside.
Heat olive oil in a large skillet or wok over medium-high heat. Add steak in a single layer and cook 2-3 minutes per side until browned. Remove from pan and set aside.
Add bell peppers and onion to the same skillet. Stir-fry for 3-4 minutes until tender-crisp. Add garlic and black pepper and cook for 30 seconds.
Return the steak to the pan. Pour in the sauce and stir until well combined. Simmer 2-3 minutes until the sauce thickens and coats everything evenly.
Optional: Garnish with green onions or sesame seeds.