01 - In a medium bowl, toss the sliced beef with 1 tablespoon soy sauce and 1 tablespoon cornstarch. Let it marinate for 10 minutes.
02 - Combine 4 tablespoons cornstarch, 2 tablespoons all-purpose flour, salt, and black pepper in a shallow dish. Dredge the marinated beef strips thoroughly in the mixture.
03 - Heat vegetable oil in a deep skillet or wok to 350°F. Fry beef strips in batches for 2 to 3 minutes until golden and crispy. Drain on paper towels.
04 - Combine soy sauce, rice vinegar, brown sugar, hoisin sauce, ketchup, minced garlic, grated ginger, chili flakes, and water in a small saucepan. Simmer over medium heat, stirring until sugar dissolves and sauce thickens slightly, about 2 to 3 minutes.
05 - Place fried beef in a clean bowl. Pour the hot sauce over and toss quickly to coat evenly.
06 - Immediately garnish with sliced green onions and toasted sesame seeds. Serve at once.