Slice the flank steak thinly against the grain into bite-sized strips. In a large bowl, toss the beef with salt, pepper, and 1/2 cup cornstarch until evenly coated. Shake off excess.
Heat the vegetable oil in a deep skillet or wok to 350°F (175°C). Working in batches, fry the beef strips for 2-3 minutes until golden and crispy, ensuring the internal temperature reaches 145°F (63°C). Drain on a paper towel-lined plate.
In a small bowl, whisk together soy sauce, rice vinegar, brown sugar, hoisin sauce, cornstarch, red pepper flakes, and water until smooth. Set aside.
In a large skillet or wok over high heat, add 1 tablespoon of the frying oil. Sauté the bell peppers and onion for 2 minutes until slightly softened. Add the garlic and ginger, cooking for another 30 seconds until fragrant.
Pour the sauce mixture into the pan with the vegetables. Bring to a simmer and cook for 1 minute until thickened. Add the crispy beef and toss quickly to coat evenly. Remove from heat immediately and serve over rice, garnished with sesame seeds and green onions.