Prepare the Beef: Slice the beef against the grain into thin strips about 1/4-inch thick. This ensures tenderness. Combine the beef with soy sauce, sesame oil, sugar, and optional oyster sauce in a bowl. Marinate for 30–60 minutes in the refrigerator.
Prepare the Vegetables: Wash all vegetables thoroughly. Slice bell peppers into thin strips, onions into long slices, and mince garlic. This keeps them crisp and flavorful during cooking.
Cook the Beef: Heat 1 tablespoon of vegetable oil in a skillet over medium-high heat. Add the beef strips and cook for 2–3 minutes per side until browned. Remove the beef and set it aside.
Cook the Vegetables: Add the remaining tablespoon of oil to the skillet. Sauté onions, garlic, and bell peppers for 2–3 minutes, keeping the vegetables slightly crisp.
Prepare the Sauce: Mix cornstarch with 2 tablespoons of cold beef broth to create a slurry. Add the remaining broth, soy sauce, black pepper, and chili flakes to the skillet. Pour in the slurry while stirring and cook until the sauce thickens.
Combine Beef and Vegetables: Return the beef to the skillet and stir to coat with the sauce. Simmer for 1–2 minutes so the flavors meld together evenly.
Serve: Garnish with fresh parsley or scallions. Serve over rice, noodles, or quinoa for a complete meal.