In a bowl, combine the sliced flank steak with soy sauce and cornstarch. Mix well and let marinate for 15-20 minutes.
Heat the sesame oil in a large skillet or wok over medium-high heat.
Add the marinated steak to the skillet, spreading it out in a single layer. Sear for about 2-3 minutes until browned, then remove and set aside.
In the same skillet, add the vegetable oil. Sauté the onions, bell pepper, garlic, and ginger. Cook for 4-5 minutes until the vegetables are tender.
Return the steak to the skillet and mix it with the vegetables. Add the oyster sauce and black pepper, stir-frying for another 2 minutes until the steak is cooked through and the sauce evenly coats the ingredients.
Remove from heat and garnish with chopped green onions before serving.
Notes
For best results, slice the steak against the grain.