Prepare the batter by mixing cornstarch, flour, egg, and water in a bowl. Coat the pork pieces with the batter.
Heat oil in a deep frying pan and fry the pork until golden and crispy. For extra crispiness, fry the pork twice.
In a saucepan, combine apple cider vinegar, sugar, ketchup, soy sauce, Worcestershire sauce, and pineapple juice. Bring to a boil, reduce heat, and simmer.
Add the cornstarch slurry to the sauce, stirring until thickened.
In a wok, stir-fry the bell peppers, onion, and pineapple for 3-4 minutes until tender but still crisp.
Toss the fried pork in the sauce and combine with the stir-fried vegetables. Serve immediately.