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.