Slice the flank steak into thin ¼-inch strips against the grain.
In a bowl, mix the beef with the beaten egg whites, salt, and 1 teaspoon cornstarch. Marinate for 30 minutes to 1 hour.
In a small bowl, whisk together the water, sugar, ketchup, hoisin sauce, soy sauce, oyster sauce, sweet chili sauce, crushed red pepper flakes, and apple cider vinegar to create the Beijing sauce.
After marinating, coat the beef slices with 2 tablespoons of cornstarch, shaking off any excess.
Heat the canola oil in a small saucepan over medium-high heat. Fry the beef slices in batches until golden brown and crispy, about 2 to 3 minutes.
In a large pan or wok, heat 2 tablespoons of the used frying oil. Stir-fry the onion and red bell pepper for 2 to 3 minutes until slightly caramelized. Add the minced garlic and cook briefly until fragrant.
Remove the vegetables and beef from the pan. Pour the prepared Beijing sauce into the pan and cook over high heat for 3 to 5 minutes until thickened.
Return the beef and vegetables to the pan. Toss everything well until coated in the glossy sauce.
Serve immediately with steamed rice or noodles.