In a large mixing bowl, combine thin slices of flank steak with baking soda, soy sauce, sesame oil, white pepper, and cornstarch. Mix thoroughly to coat the beef well, ensuring each piece is enveloped in the marinade. Let it sit for 20 minutes at room temperature to tenderize and infuse flavor as you prepare the other ingredients.

In a small bowl, whisk together the oyster sauce, Sichuan peppercorns, black pepper, Shaoxing wine, additional soy sauce, sugar, and cornstarch until smooth. Set this vibrant sauce aside.

Pour about ⅓ cup of vegetable oil into a wok or large skillet and heat it over medium-high heat until shimmering.

Carefully add half of the marinated beef to the hot oil, allowing it to sear undisturbed for 2-3 minutes. Look for a rich brown crust to form. Once seared, remove the beef to a plate and repeat with the remaining meat.

With the residual oil still in the wok, add minced garlic and sauté for about 30 seconds, or until fragrant. Quickly toss in the onions and red bell peppers, cooking them for an additional minute.

Return the seared beef to the pan, then pour in the prepared sauce. Stir continuously for 1-2 minutes, allowing the sauce to envelop the beef and vegetables as it thickens.

Before serving, drizzle a little extra sesame oil over the dish for added flavor and aroma. Serve hot over a bed of steamed rice.