In a large bowl, whisk together cornstarch, all-purpose flour, water, and egg until smooth.
Add the thinly sliced flank steak to the batter and ensure each piece is evenly coated.
Heat the vegetable oil in a deep pan to 350°F. Fry the battered beef in batches until crispy and golden, about 3 to 5 minutes. Drain on paper towels.
In a bowl, combine soy sauce, dark soy sauce, rice vinegar, brown sugar, chili flakes, and 3 tablespoons of water.
Heat sesame oil in a skillet over medium-high heat. Add minced garlic and ginger, cooking until fragrant, about 1 minute.
Pour in the sauce mixture and simmer until slightly thickened.
Add the fried beef back into the skillet with the sauce. Toss to coat evenly.
Serve warm, topped with sesame seeds and sliced green onions.
