Slice the flank steak thinly against the grain.
In a bowl, combine soy sauce, cornstarch, and black pepper; marinate the beef for 10 minutes.
Heat vegetable oil in a skillet over medium-high heat.
Add the marinated beef and stir-fry for 3-4 minutes until browned.
Remove the beef and set aside.
Add sliced onions and bell peppers to the skillet and stir-fry for 2-3 minutes.
Add minced garlic and stir for another minute.
Return the beef to the skillet and mix well with the vegetables.
Drizzle oyster sauce over the mixture and cook for another minute.
Serve warm, garnished with green onions.