Cut your flank steak into thin strips (apx 2-3 inches long by ¼ inch thick)
In a medium bowl, whisk together oyster sauce, Shao xing rice wine, soy sauce, cornstarch, 1 tsp of the black pepper, and sugar until well incorporated and the sugar has completely dissolved - Add the beef 'ribbons' and toss to combine making sure the beef is well coated - Set aside to marinate for 30 minutes at room temperature (up to 12 hours in refrigerator)
Heat the peanut oil in a wok or large skillet over high heat until shimmering - Stir the toasted sesame oil into the beef and its marinade until thoroughly combined - Add the beef and its marinade to the wok and stir fry until the beef is just browned (apx 1 minute)
Using a slotted spoon or spider, transfer the seared beef to a clean plate or bowl and set aside until needed
Return the wok/skillet to the heat (medium-high) and add the garlic and ginger - Stir fry until very fragrant (apx 30 seconds) - Add the bell pepper and continue to stir fry for 1 minute
Add the onion and continue to stir fry until onion is just tender (apx 1-2 minutes)
Add the beef and all of its juices and thoroughly combine - Add the remaining black pepper and thoroughly combine
Remove from heat - Serve immediately over steamed rice or cooked noodles along with a sunny-side-up egg on top if desired