In a large bowl, combine 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 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 a separate bowl, whisk together soy sauce, brown sugar, rice vinegar, and chili paste. Set aside.
In the same skillet, remove excess oil, leaving about 1 tablespoon. Add the sliced red and green bell peppers, sautéing for 2-3 minutes until slightly tender.
Pour the sauce over the peppers in the skillet and bring to a simmer. Add the crispy beef back into the skillet, tossing to coat everything in the sauce. Cook for an additional 2-3 minutes until heated through.
Garnish with chopped green onions and sesame seeds if desired. Serve hot.