Coarsely grind your peppercorns in a pestle and mortar, not too fine but you don’t want any whole corns either.
Tenderising your beef: Though this is not essential it real does make a huge difference. In a large bowl add the beef, ¾ of your ground peppercorns, oyster sauce, Chinese rice wine, soy sauce, a splash of sesame oil and your corn flour. Mix thoroughly and allow to tenderise for 20 minutes or overnight.
Heat your wok over a high heat. Once the wok starts to smoke immediately add the oil and then the beef, leaving any marinade in the bowl for later. Stir-fry until the outside of the beef has browned, add the garlic, green peppers, and onions, and continue stir-frying for 2 minutes. Add the remaining marinade with ½ cup of stock. Once boiling, mix the corn flour slurry together and drizzle into the sauce, stirring continuously until the sauce thickens.
Transfer immediately to a plate and sprinkle over the remaining black peppercorns.