First make the sauce. Drain the pineapple in a sieve over a bowl and keep all the juice – this will be needed for the sauce.
Put the cornflour in a small bowl and stir in 2 tbsp of pineapple juice and mix together so there are no lumps.
Add the mixed cornflour to the remaining juice, along with the water, soy sauce, vinegar, sugar, ketchup and chilli flakes, stir and set aside.
Heat some oil in a frying pan and add the chicken (this might need to be done in batches depending on the size of your frying pan) and fry until colour on all sides. Set aside.
Fry the peppers and onion granules (or chunks) for a few mins until slightly softened. if using, add the chestnuts and stir. Return the chicken to the pan. Add the garlic, ginger, pineapple chunks and stir for a minute.
Give the sauce a stir then add to the pan with the chicken and vegetables, stir well and season and then simmer whilst you cook the rice.
Cook the rice according to the packets instructions.
Check the chicken is cooked, serve with the rice and shop bought prawn crackers.