- Toss the sliced flank steak with cornstarch until evenly coated and let it sit for about 15 minutes. 
- Heat the vegetable oil in a large skillet or wok over medium-high heat. 
- Add the beef in a single layer and cook for about 2-3 minutes on each side until browned and crispy. Remove from skillet and set aside. 
- Add the bell pepper, broccoli, ginger, and garlic to the same skillet and stir-fry for about 3-4 minutes until tender-crisp. 
- In a separate bowl, whisk together the orange juice, soy sauce, brown sugar, rice vinegar, sesame oil, and red pepper flakes. 
- Pour the sauce into the skillet with the vegetables and bring to a simmer. 
- Return the crispy beef to the skillet and toss everything together until the beef is coated in the sauce and heated through, about 2 minutes. 
- Serve hot, garnished with chopped green onions.