Begin by slicing the beef sirloin or flank steak thinly against the grain into strips about 1/4 inch thick. Set the beef aside to come to room temperature.

In a mixing bowl, combine cornstarch, all-purpose flour, baking powder, salt, and pepper. Gradually add the beaten egg and water, whisking until smooth.

In a large frying pan, heat vegetable oil over medium-high heat until shimmering. Test with a drop of batter to check if ready.

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

In a separate wok, heat sesame oil and sauté garlic, ginger, onion, and bell pepper for 2 minutes until tender.

Stir in soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar. Let simmer for about 2 minutes.

Add the fried beef to the sauce and toss to coat. Stir-fry for an additional 2-3 minutes.

Garnish with red chilies and chopped green onions. Serve immediately.