To marinate the pork, in a bowl, combine the pork with salt, white pepper, Shaoxing wine, and egg white. Mix well and let marinate while preparing the other ingredients.
To prepare the sauce, in a bowl, whisk together water, ketchup, rice vinegar, sugar, and salt until smooth.
To coat the pork, mix the pork with half of the cornstarch and let rest for 5 minutes. Coat again with the remaining cornstarch.
To fry the pork, in a pot over high heat, heat the cooking oil to 350°F (177°C). Fry the pork in batches for about 3 minutes until cooked through. Increase the oil temperature to 375°F (191°C) and fry again for about 1 minute until golden and crispy.
To stir-fry and finish, in a wok over high heat, add the cooking oil and stir-fry the onion and bell peppers for about 30 seconds, then remove. Reduce the heat to medium and add the sauce mixture and the cornstarch slurry. Stir until the sauce thickens. Return the vegetables and pork to the wok, add pineapple, and toss until evenly coated.