Slice the beef thinly against the grain into strips about 1/4 inch thick and set aside.
In a mixing bowl, combine cornstarch, flour, baking powder, salt, pepper, egg, and water to make a smooth batter.
Heat vegetable oil in a skillet or wok. Dip beef strips in batter and fry in batches for 3-4 minutes until golden brown. Drain on paper towels.
In the same pan, add sesame oil and sauté garlic, ginger, onion, and bell pepper for about two minutes.
Stir in soy sauce, rice vinegar, hoisin sauce, chili sauce, and sugar. Simmer for about two minutes.
Add the crispy beef back into the pan and toss everything together for 2-3 minutes.
Garnish with sliced red chilies and chopped green onions. Serve immediately.