Slice the beef sirloin or flank steak against the grain into thin strips, about ¼-inch thick. Set aside to come to room temperature for about 10 minutes.

In a large bowl, whisk together the cornstarch, all-purpose flour, baking powder, salt, and pepper. Create a well, add the egg, and gradually incorporate water until smooth.

Heat vegetable oil in a frying pan over medium-high heat (about 350°F). Dip beef strips in the batter, ensuring they are well coated. Fry in batches for 3-4 minutes until golden brown and crispy. Drain on paper towels.

In a wok over medium heat, add sesame oil and sauté minced garlic, ginger, onion, and bell pepper for about 2 minutes until fragrant.

Add soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar to the wok. Stir thoroughly and simmer for 2 minutes.

Carefully add the crispy beef to the sauce, tossing to coat. Stir-fry for 2-3 minutes, allowing the sauce to cling to the beef.

Transfer to a serving dish, garnish with sliced red chilies and chopped green onions, and serve hot with steamed rice or noodles.