Slice the beef sirloin or flank steak thinly against the grain into strips.

In a medium mixing bowl, combine cornstarch, all-purpose flour, baking powder, salt, and pepper. Whisk together until blended. Add the egg and water, mix until smooth.

Heat vegetable oil in a frying pan or wok over medium-high heat until it reaches about 350°F (175°C).

Dip the beef strips into the batter, coating evenly, and fry in batches for 3-4 minutes until golden brown and crispy. Drain on paper towels.

In a wok, heat sesame oil over medium heat. Add garlic, ginger, onion, and bell pepper; sauté until onion is translucent.

Pour in soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar. Stir and simmer for 2 minutes until the sauce thickens.

Add the crispy beef strips to the sauce and toss to coat, stir-fry for an additional 2-3 minutes.

Transfer to a serving dish and garnish with sliced red chilies and green onions. Serve immediately over rice or noodles.