In a large bowl, combine the cornstarch, flour, salt, black pepper, garlic powder, and onion powder. Add the sliced flank steak and toss until well coated.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the coated beef in batches, frying until golden brown and crispy, about 3-4 minutes per batch. Remove and drain on paper towels.

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

In a small bowl, whisk together the soy sauce, rice vinegar, brown sugar, and red pepper flakes. Pour this sauce over the stir-fried vegetables and bring to a simmer.

Add the crispy beef back into the skillet, tossing everything together until the beef is well coated in the sauce. Cook for an additional 2 minutes.

Serve the crispy chili beef over cooked white rice and garnish with chopped green onions and sesame seeds.