Place the chicken breast chunks in a bowl and sprinkle on the cornflour, salt, and pepper. Mix with your hands to cover the chicken in the cornflour.
Heat the oil in a wok or large frying pan over a high heat.
When the oil is hot, add in the chicken. Cook on a high heat, moving the chicken around occasionally with a spatula, until the chicken is golden on all sides (about 5-6 minutes).
While the chicken is cooking, make the stir-fry sauce by combining the pineapple juice from the tin of pineapple with the brown sugar, soy sauce, and tomato puree in a bowl. Mix together and put to one side.
Add in the onions and peppers to the wok, and fry with the chicken for 2 minutes.
Add the pineapple chunks, from the tinned pineapple, into the wok and cook for a further two minutes.
Stir in the garlic and ginger and cook for a further minute.
Add the stir-fry sauce you made earlier to the wok.
Continue cooking on high, allowing the sauce to bubble and reduce down, whilst stirring occasionally. It should take about 5-6 minutes on a medium-high heat to reduce down and start making the chicken look darker and caramelized.
Once cooked, turn off the heat and serve over boiled rice. Sprinkle with coriander and serve.
