Crush whole black peppercorns using a mortar and pestle or a rolling pin in a ziplock bag for a coarse texture.

In a bowl, whisk together oyster sauce, Shaoxing wine, soy sauce, toasted sesame oil, and most of the crushed black pepper. Stir in potato starch until smooth.

Marinate cut beef tenderloin in the sauce for about 10 minutes while prepping vegetables.

Preheat a frying pan or wok on medium-high heat for about 2 minutes. Add vegetable oil and sauté chopped garlic until fragrant.

Add marinated beef to the pan, spreading it in a single layer. Cook for 2-3 minutes on one side, then flip and cook for another 2-3 minutes until cooked through.

Add onion squares and sliced bell peppers. Stir-fry for 2-3 minutes until the onions are translucent and the peppers are tender.

Pour in the remaining black pepper sauce, tossing to coat. Stir-fry for 1-2 minutes until the sauce thickens.

Serve immediately, garnishing with extra crushed black pepper if desired, alongside steamed rice or stir-fried noodles.