In a covered container, stir together the cornflour and soy sauce until will combined. Add in the chicken and shake until well covered (this is called velveting your meat, it seals in the juices so your meat is tender and does not overcook).
In a deep frying pan heat the oil. Add the velveted chicken and cook until browned. Add water, onion, capsicum and carrot. Stir in the sweet and sour sauce. Cover and simmer until chicken is cooked though, about 20 minutes.
Stir in pineapple chunks and soy sauce. Allow to cook until heated though, about 5 minutes. Serve over rice.
Frozen Chicken Breasts?
I prefer to keep my chicken breast frozen. I thaw them on the bench for about 20 minutes, then with a sharp serrated knife, I cut off thin 2-3mm slices at a time. This way it can cook from frozen and I prefer the texture it gives to the meat.