Place chicken chunks into a large mixing bowl and toss with cornflour, salt, and pepper until fully coated.
Heat oil in a wok or large frying pan over high heat until shimmering.
Add the chicken pieces to the hot oil and stir-fry for 5 to 6 minutes until golden brown on all sides.
In a small mixing bowl, combine pineapple juice, brown sugar, soy sauce, and tomato puree to create the base sauce.
Add the chopped onion and bell pepper into the wok with the chicken and fry for 2 minutes.
Pour the prepared sauce and pineapple chunks into the wok.
Continue cooking and tossing everything together for 3 to 5 minutes, allowing the sauce to bubble and thicken into a sticky glaze that coats the chicken.