Prepare the beef: In a bowl, combine the thinly sliced flank steak with 1 tablespoon soy sauce and 1 tablespoon cornstarch. Toss well until each piece is evenly coated. Let it rest for about 10–15 minutes to tenderize and help the coating adhere, which will also give the beef a better texture when cooked.

Make the sauce: In a separate bowl, mix together 1/4 cup soy sauce, oyster sauce, hoisin sauce, rice vinegar, brown sugar, beef broth, black pepper, and crushed red pepper flakes if using. Stir thoroughly until the sugar is dissolved and the sauce is well combined. Set aside for later use.

Sear the beef: Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat. Once the oil is hot, add the marinated beef in a single layer (cook in batches if needed to avoid overcrowding). Sear for 2–3 minutes until browned but not fully cooked through. Remove the beef from the pan and set aside.

Cook the vegetables: In the same pan, add the sesame oil, then sauté the minced garlic, ginger, onion, and bell peppers. Stir frequently and cook for about 3–5 minutes until the vegetables are slightly tender but still crisp, allowing them to retain some crunch and vibrant color.

Combine everything: Return the cooked beef to the pan with the vegetables. Pour in the prepared sauce and toss everything together to coat evenly. Let it simmer for 2–3 minutes so the flavors meld and the beef finishes cooking through.

Thicken the sauce (optional): If you prefer a thicker sauce, stir in the cornstarch slurry and cook for another 1–2 minutes until the sauce becomes glossy and slightly thickened. Stir continuously to avoid lumps and ensure even consistency.

Serve hot: Remove from heat and serve immediately over steamed white rice, fried rice, or noodles. Garnish with extra black pepper or sliced green onions if desired for added freshness and flavor.