In a small bowl, whisk together the tomato ketchup, vinegar, brown sugar, soy sauce, and the reserved pineapple juice. Set aside.
Place the chicken pieces in a large zip-lock bag or bowl. Add the onion powder, garlic powder, salt, pepper, and cornflour. Seal the bag (or stir in the bowl) and toss until the chicken is evenly coated.
Heat 1 tablespoon of oil in a large frying pan over medium-high heat. In a shallow bowl, whisk the eggs.
Working in batches, dip each chicken piece into the egg, letting any excess drip off, then place into the hot pan. Avoid overcrowding.
Fry the chicken for 2-3 minutes per side, until golden and crispy (it doesn’t need to be fully cooked yet). Transfer to a plate and repeat with the remaining chicken, adding more oil if needed.
Wipe out any excess oil from the pan. Return all the chicken to the pan along with the chopped capsicum and pineapple pieces. Pour over the prepared sauce and stir to coat.
Cover with a lid, reduce heat to medium-low, and cook for 7-8 minutes. Remove the lid and cook for another 2-3 minutes until the sauce is thick, sticky, and the chicken is cooked through.
Spoon the sweet and sour chicken over cooked rice. Sprinkle with sesame seeds and sliced spring onions.