Step 1: In a medium bowl, combine the thinly sliced flank steak with 1 tablespoon cornstarch, 1 tablespoon soy sauce, 1 teaspoon sesame oil, and 1/4 teaspoon black pepper. Mix well to coat all the beef, then let it marinate for at least 15-20 minutes at room temperature.
Step 2: While the beef marinates, prepare the sauce. In a small bowl, whisk together the beef broth, 2 tablespoons soy sauce, oyster sauce, brown sugar, 1 teaspoon cornstarch, and 1/4 teaspoon black pepper until smooth. Set aside.
Step 3: Heat 1 tablespoon of vegetable oil in a large wok or a heavy-bottomed skillet over high heat until it just begins to smoke. Add half of the marinated beef in a single layer and stir-fry for 1-2 minutes until browned and mostly cooked through. Remove the cooked beef to a clean plate. Add the remaining 1 tablespoon of vegetable oil and cook the rest of the beef, then remove to the same plate.
Step 4: Add the sliced green bell pepper, red bell pepper, and yellow onion to the hot wok. Stir-fry for 3-4 minutes until the vegetables are crisp-tender. Add the minced garlic and grated ginger and stir-fry for another minute until fragrant.
Step 5: Return the cooked beef to the wok with the sautéed vegetables. Give the prepared sauce mixture a quick whisk again (as cornstarch can settle) and pour it over the beef and vegetables. Stir continuously to combine everything. The sauce will quickly thicken and become glossy, coating all the ingredients, in about 1-2 minutes.
Step 6: Remove from heat immediately and serve your Sizzling Chinese Pepper Steak hot over a bed of steamed white rice. Enjoy!