Prepare the Ingredients: Slice the beef against the grain into thin strips. Slice the bell peppers and onion into uniform strips. Mince the garlic and grate the ginger.
Marinate the Beef: In a bowl, mix 2 tablespoons of soy sauce and 1 teaspoon of sesame oil. Toss the beef strips in the marinade and let sit for 10 minutes.
Make the Sauce: In a small bowl, whisk together soy sauce, beef broth, oyster sauce, brown sugar, and the cornstarch slurry. Set aside.
Cook the Beef: Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Stir-fry the beef strips for 2-3 minutes until browned. Remove from the skillet and set aside.
Stir-Fry Vegetables: Add the remaining vegetable oil to the skillet. Stir-fry the bell peppers, onion, garlic, and ginger for 3-4 minutes until tender-crisp.
Combine and Simmer: Return the beef to the skillet. Pour in the sauce and stir to coat everything evenly. Simmer for 2-3 minutes until the sauce thickens.
Serve and Enjoy: Remove from heat and garnish with optional red chili flakes or sesame seeds. Serve hot over steamed rice or noodles.