Begin by slicing the beef sirloin or flank steak thinly against the grain into strips, approximately 1/4 inch thick. Set aside.

In a mixing bowl, combine cornstarch, all-purpose flour, baking powder, salt, and pepper. Whisk until even. Add egg and water, mixing until smooth.

In a frying pan, heat vegetable oil to 350°F (180°C). Test with a spoonful of batter; it should sizzle.

Coat the beef strips in the batter and fry in batches for 3-4 minutes until golden brown. Drain excess oil on paper towels.

Heat sesame oil in a separate pan, and sauté garlic, ginger, onion, and bell pepper for about 2 minutes.

Mix in soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar. Simmer for 2 minutes.

Add crispy beef strips back into the sauce and toss to coat. Stir-fry for an additional 2-3 minutes.

Remove from heat and garnish with red chilies and green onions. Serve hot.