1. Marinate the Beef

• In a medium bowl, combine the soy sauce, cornstarch, and sesame oil.
• Add the thinly sliced flank steak and toss to coat. Let it marinate for 15 minutes while you prepare the other ingredients.

2. Prepare the Sauce

• In a small bowl, whisk together soy sauce, oyster sauce, rice vinegar, brown sugar, and cornstarch. Set aside.

3. Cook the Beef

• Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat.
• Add the marinated beef in a single layer and sear for 1-2 minutes per side until browned.
• Remove the beef from the pan and set aside.

4. Stir-Fry the Vegetables

• Add the remaining tablespoon of oil to the wok.
• Add the minced garlic, ginger, and crushed red pepper flakes (if using) and stir-fry for 30 seconds until fragrant.
• Add the bell peppers and onion to the wok. Stir-fry for 3-4 minutes until the vegetables are slightly softened but still crisp.

5. Combine and Cook

• Return the beef to the wok with the vegetables.
• Pour the prepared sauce over the mixture and toss to coat evenly.
• Stir-fry for another 2-3 minutes until the sauce thickens and everything is heated through.

6. Serve

• Remove the wok from heat and garnish with sliced green onions and sesame seeds, if desired.
• Serve hot over steamed rice or noodles.