In a bowl, combine the sliced flank steak with soy sauce and cornstarch. Mix well and let marinate for at least 30 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the marinated beef in a single layer. Cook for about 3-4 minutes until crispy and browned, flipping halfway through. Remove the beef and set aside.
In the same skillet, add bell pepper, onion, garlic, and ginger. Stir-fry for 2-3 minutes until the vegetables are tender.
Return the beef to the skillet. Add hoisin sauce, rice vinegar, and sesame oil. Toss everything together and cook for another 2 minutes until heated through.
Serve hot, garnished with chopped green onions and sesame seeds if desired.