Place the sliced flank steak in a large bowl and toss it with cornstarch until each slice is evenly coated. Let sit for 15 minutes.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the coated beef in a single layer and sear for 2-3 minutes on each side until browned and crispy. Remove from heat and set aside.

In the same skillet, add the red and green bell peppers, onion, and garlic. Stir-fry for 3-4 minutes, until the veggies are tender-crisp.

In a small bowl, whisk together soy sauce, brown sugar, rice vinegar, ginger, and red pepper flakes. Pour this mixture over the sautéed vegetables and stir to combine.

Return the cooked beef to the skillet and toss everything together until heated through and the beef is glazed with the sauce, approximately 2 minutes.

Serve hot, garnished with chopped green onions over steamed rice or noodles.