Slice the flank steak thin against the grain, no thicker than 5mm. Toss with the soy sauce, cornstarch, and Shaoxing wine. Let it sit 10 minutes while you cut the vegetables.
Stir the oyster sauce, soy sauce, sugar, and water together for the sauce.
Heat 1 tablespoon of oil in a wok over the highest heat your stove has, until it just starts to smoke. Lay the beef in a single layer and leave it alone for 60 seconds before stirring. Cook 2 minutes total, until browned but not quite cooked through, then scoop it out.
Add the remaining oil, then the crushed peppercorns. Toast them in the oil for 20 to 30 seconds until fragrant. This step is the whole dish.
Add the onion and bell pepper. Stir fry 2 minutes, keeping some crunch. Add the garlic and cook 30 seconds more.
Return the beef with any juices, pour in the sauce, and toss for about a minute until everything is glossy and the beef is just cooked. Serve immediately over rice.