In a 12-inch skillet over high, heat 2 tablespoons oil until barely smoking. Add the onion and bell pepper; cook, stirring often, until the onion begins to wilt, about 2 minutes. Add the peppercorns and cook, stirring, until the mixture is fragrant and the onion begins to brown at the edges, 1 to 2 minutes. Transfer to a plate and set aside.
In the same skillet over high, heat the remaining 1 tablespoon oil until barely smoking. Add the beef in an even layer and cook, without stirring, until well browned on the bottom and the pieces release easily from the skillet, 2 to 3 minutes. Stir in the vegetable-peppercorn mixture. Add the oyster and soy sauces; cook, stirring, until the beef is tender and the sauce clings to the meat, 1½ to 3 minutes.
Optional garnish: Chopped cilantro OR lime wedges OR a combination
