Season the pork with salt and pepper. Whisk the egg and water in a bowl, then stir in the cornstarch to make a thick batter.
Coat the pork pieces in the batter. Heat the oil in a large skillet to about 350 F (175 C).
Fry the pork in batches for 3 to 4 minutes, turning occasionally, until golden and cooked through. Transfer to a wire rack.
Make the Sauce:
Whisk the ketchup, rice vinegar, pineapple juice, brown sugar, and soy sauce in a bowl. Stir the cornstarch and water together separately.
Pour the sauce into the skillet or wok over medium-high heat. Add the cornstarch slurry and cook for 1 to 2 minutes, until glossy and lightly thickened.
Finish:
Add the peppers, onion, and pineapple to the sauce. Stir-fry for 2 to 3 minutes, keeping the vegetables slightly crisp.
Return the fried pork to the pan and toss for 30 to 60 seconds, just until coated. Serve immediately with rice.
Notes
Frying in batches keeps the oil hot and the pork crisp.
Add the pork only after the sauce and vegetables are ready.
Chicken or firm tofu can be substituted for the pork using the same batter.