Place the cornflour in a medium bowl and season it well with salt and pepper.
In a high sided skillet heat the vegetable oil and gently fry the cornflour chicken on all sides until golden brown. Remove the chicken to a plate lined with a paper towel. It’s okay if your chicken isn't cooked all the way through at this stage. It gets put into the sauce to finish cooking.
After you've fried the chicken, you will likely (hopefully) have brown bits stuck to the bottom of your skillet. These bits are a flavour explosion, so we want to make sure we get them scraped up. To do that add in the sliced onion. The moisture from the onion will soften those bits and make them easy to scrape up with a spatula or wooden spoon.
Add in the chopped bell peppers to get them softened. Once the onions are soft and the peppers are in the pan, add the sauce in.
To make the sauce
Add all the ingredients to a bowl or measuring jug and whisk. Pour the sauce into the skillet and let it come to a gentle simmer. When adding the pineapple, reserve the juice and add some in according to your tastes.
The sauce will thicken as it simmers and once you add the chicken back in.
Add the chicken back to the sauce. Continue to simmer in the sauce until the chicken is through. This should only be about another 5-6 minutes.