In a mixing bowl, combine the cornstarch, flour, salt, black pepper, garlic powder, and onion powder. Toss the sliced flank steak in the mixture until well coated.

Heat the vegetable oil in a large skillet 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 a separate bowl, whisk together the soy sauce, brown sugar, rice vinegar, and chili paste. Set aside.

In the same skillet, add the sliced bell peppers and onion. Stir-fry for about 2-3 minutes until slightly tender.

Return the crispy beef to the skillet and pour the sauce over the mixture. Toss everything together to coat evenly and heat through for another 2 minutes.

Serve immediately, garnished with chopped green onions and sesame seeds if desired.