Mix prawns, red chilli sauce and crushed garlic in a bowl.
Add prawns to the wok, mix well and cook for 4-5 minutes. Transfer them into another bowl.
Cut green and red capsicum in small pieces.
Place the same wok on heat, add remaining oil, add capsicum pieces, pineapple cubes, chicken stock, salt and castor sugar.
Roughly chop red chillies and add to the wok and cook for 4-5 minutes. Add cornstarch slurry and the prawns and mix well. Add vinegar and mix well. Add coriander leaves and mix well and cook till the prawns are done.
Put the prawns with pineapple into the pineapple boat and serve immediately.