Step 1: Prepare the beef. In a medium bowl, combine the thinly sliced beef with marinade ingredients (1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, 1/2 tsp baking soda). Mix well and let marinate at room temperature for at least 15-30 minutes, or in the refrigerator for up to 1 hour.
Step 2: Prepare the sauce. In a small bowl, whisk together all sauce ingredients (1/2 cup beef broth, 2 tbsp soy sauce, 1 tbsp oyster sauce, 1 tbsp rice vinegar, 1 tbsp brown sugar, 1 tsp cornstarch, 1/2 tsp black pepper) until the cornstarch is dissolved. Set aside.
Step 3: Cook the beef. Heat 1 tbsp vegetable oil in a large wok or skillet over high heat until shimmering. Add half of the marinated beef in a single layer and stir-fry for 1-2 minutes until browned but not fully cooked. Remove beef from the pan and set aside. Add the remaining 1 tbsp oil and cook the rest of the beef, then remove and set aside with the first batch.
Step 4: Sauté aromatics and vegetables. Add the garlic and ginger to the hot pan and stir-fry for 30 seconds until fragrant. Add the sliced green bell pepper, red bell pepper, and yellow onion. Stir-fry for 2-3 minutes until vegetables are crisp-tender.
Step 5: Combine and sauce. Return the cooked beef to the pan with the vegetables. Give the prepared sauce a quick whisk again (as cornstarch may settle) and pour it over the beef and vegetables.
Step 6: Thicken and serve. Stir continuously for 1-2 minutes until the sauce thickens and coats all the ingredients. Cook until beef is fully cooked through and vegetables are still slightly crisp. Do not overcook.
Step 7: Serve immediately. Transfer the Chinese Pepper Steak to serving plates and serve hot over steamed white rice or noodles.