Start by slicing the beef sirloin or flank steak against the grain into thin strips, aiming for about 1/4 inch thick.

Place the sliced beef in a bowl and sprinkle with a pinch of salt and pepper to enhance the flavor. Set aside to marinate briefly.

In a mixing bowl, combine cornstarch, all-purpose flour, baking powder, salt, and pepper. Whisk together.

Add the egg and gradually pour in water while whisking to achieve a smooth, slightly thick consistency.

Heat vegetable oil in a deep skillet or wok over medium-high heat until shimmering, approximately 350°F (175°C).

Dip the marinated beef strips into the batter, allowing excess to drip off. Fry the beef in batches for 3-4 minutes until golden brown.

Transfer the cooked strips to a paper towel-lined plate to absorb excess oil.

In the same pan, reduce heat to medium and add a drizzle of sesame oil. Add minced garlic, minced ginger, diced onion, and bell pepper, sauté for about 2 minutes.

Pour in soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar into the pan with sautéed vegetables. Stir well and simmer for 2 minutes.

Add the crispy beef strips into the sauce mixture, tossing gently to ensure each piece is coated. Cook for an additional 2-3 minutes.

Remove from heat and garnish with sliced red chilies and finely chopped green onions.

Serve immediately over a bed of steamed rice or noodles.