In a bowl, combine the cornstarch and all-purpose flour.
In another bowl, beat the egg and water together.
Dip each piece of pork into the egg mixture, then coat with the flour mixture. Shake off any excess flour.
Heat vegetable oil in a deep pan or wok over medium-high heat.
Fry the pork pieces in batches until they are golden brown and crispy, about 4-5 minutes per batch.
Remove the pork from the oil and drain on paper towels.
In a small bowl, mix together the ketchup, rice vinegar, sugar, and soy sauce.
In a separate bowl, mix the cornstarch with water to create a slurry.
In a large skillet or wok, heat a tablespoon of vegetable oil over medium-high heat.
Add the bell pepper and onion, and stir-fry for about 3-4 minutes until they start to soften.
Add the pineapple chunks (if using) and cook for another 2 minutes.
Pour the sauce mixture into the skillet with the vegetables.
Bring the sauce to a simmer, then add the cornstarch slurry to thicken the sauce. Stir until the sauce thickens.
Add the fried pork pieces to the skillet and toss everything together until well combined and heated through.
Enjoy your meal!