Prep the chicken. Pat the chicken dry with paper towels.
In a bowl, mix soy sauce, Shaoxing wine, sesame oil, egg white, salt, and white pepper. Add chicken and toss to coat. Let it marinate for 15 minutes while you prepare the vegetables and sauce.
Make the sauce. In a small bowl or measuring cup, whisk ketchup, rice vinegar, sugar, soy sauce, oyster sauce, pineapple juice, and cornstarch until smooth.
Set aside.
Chop the veggies. Cut the bell peppers and onion into 1-inch pieces. Mince the garlic and grate the ginger. Keep the pineapple chunks ready and drained if using canned.
Coat the chicken. In a shallow bowl, combine cornstarch and flour.
Add the marinated chicken pieces, letting excess marinade drip off. Toss to coat evenly. Shake off extra starch so the coating is light, not cakey.
Heat the oil. Pour about 1 inch of oil into a deep skillet or wok.
Heat over medium-high until it reaches 350°F (175°C). If you don’t have a thermometer, dip a wooden chopstick—the oil should bubble steadily around it.
Fry in batches. Add chicken in a single layer, leaving space between pieces. Fry 4–5 minutes, turning once, until golden and cooked through.
Transfer to a wire rack or paper towels. Repeat with remaining chicken.
Stir-fry the vegetables. Discard all but 1 tablespoon of the frying oil. Return the pan to medium-high heat.
Add onion, bell peppers, and a pinch of salt. Stir-fry 2–3 minutes until crisp-tender. Add garlic and ginger; cook 30 seconds until fragrant.
Add pineapple and sauce. Stir in pineapple chunks.
Whisk the sauce again (the cornstarch settles), then pour it into the pan. Cook, stirring, until the sauce thickens and turns glossy, about 1–2 minutes.
Toss with chicken. Add fried chicken to the pan and toss to coat every piece. If the sauce gets too thick, splash in a tablespoon of water to loosen.
Taste and adjust with a pinch of salt or an extra dash of vinegar if you like it tangier.
Serve. Spoon over hot rice. Garnish with sesame seeds and scallions if you want a little crunch and color. Serve immediately for the best texture.