In a bowl, combine the sliced flank steak, soy sauce, cornstarch, and vegetable oil. Mix well to coat the beef evenly. Let it marinate for at least 10 minutes to enhance the flavor and tenderness.

In a small bowl, whisk together soy sauce, oyster sauce, fish sauce, and sugar until the sugar dissolves. Set aside.

Heat a wok or large skillet over high heat. Add 2 tablespoons of vegetable oil and swirl to coat the pan.

Once the oil is hot, add the marinated beef in a single layer. Let it sear for about 1-2 minutes without stirring to develop a nice crust.

Stir-fry the beef for an additional 2-3 minutes until it is browned and cooked through. Remove the beef from the pan and set aside.

In the same pan, add the minced garlic and sliced Thai chili peppers. Stir-fry for about 30 seconds until fragrant, being careful not to burn the garlic.

Add the sliced bell pepper and snap peas. Stir-fry for 2-3 minutes until the vegetables are tender-crisp.

Return the cooked beef to the pan with the vegetables. Pour the prepared sauce over the mixture and toss everything together to coat evenly. Cook for an additional 1-2 minutes until heated through.

Add the fresh Thai basil leaves and stir until wilted. Remove from heat.

Serve the Thai basil beef hot over steamed jasmine rice or rice noodles. Enjoy your delicious meal!