Start by slicing the flank steak thinly against the grain. Add the beef to a bowl with egg whites, salt, and cornstarch, then mix well. Let it rest in the fridge for up to an hour so the beef stays tender and absorbs flavor.
In a separate bowl, whisk together water, sugar, ketchup, hoisin sauce, soy sauce, oyster sauce, sweet chili sauce, red pepper flakes, and apple cider vinegar. Set it aside so it’s ready when cooking moves fast.
Coat the marinated beef with cornstarch until evenly covered. Heat oil in a pan and fry the beef in small batches until crispy and golden. Remove and drain on paper towels.
Using a small amount of oil, sauté onions and bell peppers until slightly softened and lightly caramelized. Add minced garlic and cook just until fragrant.
Pour the sauce into the pan and let it bubble until thick and glossy. Add the crispy beef back in and toss everything together until fully coated. Serve immediately while hot.