Preheat your oven to 200°C (390°F).
Place the chicken pieces in a bowl, sprinkle with salt, and pour in the beaten egg. Stir to ensure each piece is coated thoroughly. Allow any excess egg to drain away.
Transfer the egg-coated chicken, a handful at a time, into the prepared bag of cornflour. Shake the bag well to coat the chicken evenly. Try to trap air inside the bag to create a balloon effect for better coating. Once coated, discard any remaining cornflour.
Heat oil in a large frying pan over high heat. Working in batches if necessary, briefly sear the chicken pieces for about 2 minutes, turning to develop a light golden crust. The interior should remain raw at this stage.
Move the chicken to a baking sheet in a single layer. Lightly spray with cooking oil and place in the oven. Bake for 15–20 minutes, or until the coating becomes crisp and golden.
While the chicken bakes, discard any leftover oil from the pan and wipe clean with paper towels. Set the pan over medium-high heat and add ½ tablespoon of oil. Sauté the garlic for 20 seconds.
Add the chopped onion and cook for approximately 2 minutes until slightly softened. Introduce the bell pepper pieces and cook for another minute.
Pour in the sauce mixture ingredients: vinegar, sugar, ketchup, pineapple juice, soy sauce, Worcestershire sauce, oyster sauce, and salt. Stir well until the sugar dissolves and the sauce begins to simmer.
Add the prepared cornflour slurry to the sauce, stirring continuously as it thickens into a syrup-like consistency—this should take around 3 minutes.
Stir in the pineapple chunks just until warmed through. Remove the pan from heat.
Once the chicken is crisp and ready, add it directly into the pan with the sauce. Toss gently to ensure every piece is coated evenly.
Serve hot alongside steamed rice, or opt for cauliflower rice as a lighter alternative. Garnish with sliced scallions if desired.