Combine chicken, a pinch of salt, and white pepper in a bowl. Coat chicken with cornstarch, then dip in beaten egg to cover fully. Heat vegetable oil in a deep pan or wok and fry chicken in batches until golden and crisp. Drain on paper towels.
Drain excess oil from the wok, leaving about a tablespoon. Stir-fry chopped garlic until fragrant. If desired, add bell pepper, onions, and pineapple chunks; cook for 2–3 minutes until crisp-tender.
Combine dark vinegar, white vinegar, ketchup, oyster sauce, light soy sauce, and sesame oil in a bowl. Mix well. Thicken in the vegetable pan with cornstarch slurry, stirring until glossy and evenly coating the vegetables.
Toss fried chicken in the pan with the sauce for 1–2 minutes to coat. Serve on a plate, topped with chopped spring onions.
