Put the peppercorns in a mortar and use a pestle to crush the pepper into large pieces. You don't want any whole peppercorns, but it should be very coarse.
Put the beef into a bowl and add 3/4 of the black pepper in with it along with the oyster sauce, Shaoxing, potato starch, soy sauce and sesame oil. Mix thoroughly and let this marinate for at least 15 minutes.
Heat a wok or cast iron skillet over high heat until hot. Add the oil, and then immediately add the beef, leaving any marinade in the bowl for later. Fry one side until browned, and then use a hard spatula to stir-fry until the outside of the beef is cooked and browned. Add the garlic, bell peppers, and onions, and continue stir-frying. Add the remaining marinade and stir-fry until the onions are cooked.
Transfer immediately to a plate and garnish with the remaining black pepper.
Serve with some steamed white rice.