Slice beef into thin slices against the grain. Cut each strip across the grain about 1 inch long so you have small thin slices. Place in a bowl and add 4 tsp soy sauce, 1 tbsp rice wine, 1 tsp cornstarch, and black pepper. Mix well to coat the beef.

In a small bowl, whisk together 3 tbsp soy sauce, 1 tbsp water, and 2 tsp cornstarch. Set aside.

Heat a wok or large skillet over high heat. Add 1 teaspoon of vegetable oil and swirl to coat. Add the beef in a single layer, cooking undisturbed for about 20 seconds to brown. Stir the beef and cook for another 2 minutes until browned. Remove the beef from the wok and set aside on a plate.

Add the remaining 1 teaspoon of vegetable oil to the wok. Add the sliced onion and bell pepper and stir-fry for about 4 to 5 minutes, until tender-crisp.

Return the cooked beef to the wok with the vegetables. Pour the prepared sauce over the mixture and add the crushed red pepper flakes (if using). Stir-fry for about 30 seconds, or until the sauce has thickened and coats the ingredients.