1. Slice the Beef: Start by thinly slicing the beef against the grain. This ensures the strips will be tender and easy to chew, perfectly complementing the crispy coating.

2. Make the Batter: In a bowl, combine the cornstarch, flour, baking powder, salt, and pepper. Whisk in the egg and water until you have a smooth, pourable batter. The consistency should be thick enough to cling to the beef, but thin enough to allow for even coverage.

3. Fry the Beef: Heat a generous amount of vegetable oil in a frying pan or wok over medium-high heat. Once the oil is hot, dip the beef strips into the batter, allowing any excess to drip off. Gently lower the coated beef into the hot oil and fry for 3-4 minutes, or until golden brown and crispy. Work in batches to avoid overcrowding the pan, which can lower the oil temperature and result in soggy beef.

4. Prepare the Sauce: While the beef is frying, heat the sesame oil in a wok or large pan over medium heat. Sauté the garlic, ginger, onion, and bell pepper until fragrant and softened. Then, stir in the soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar. Let the sauce simmer for a few minutes, allowing the flavors to meld and the consistency to thicken slightly.

5. Coat the Beef: Once the beef is crispy and golden, transfer it to the pan with the prepared sauce. Toss the beef to coat it evenly, ensuring every morsel is infused with the bold, spicy-sweet flavors.

6. Garnish and Serve: Finish the dish by garnishing with sliced red chilies (if using) and chopped green onions. Serve the Crispy Chilli Beef immediately, either over a bed of steamed rice or alongside your favorite noodles.