Place the chicken chunks in a bowl and sprinkle on the cornstarch, salt & pepper. Heat the oil in a large pan on high. When the oil is hot, add in the chicken. Cook, moving the chicken around occasionally, until the chicken is golden on all sides. Add in the carrots, onions and peppers, and fry with the chicken for a 1 minute. Drain the pineapple (reserving the juice) and dry the pieces on a piece of kitchen towel. Add the pineapple into the pan and cook for a further two minutes. Stir in the garlic and ginger and cook for a further minute. Combine the reserved pineapple juice, brown sugar, soy sauce and the tomato paste in a bowl. Mix together, then add to the pan. Continue cooking on high, allowing the sauce to bubble and reduce down, stirring occasionally. It should take about 5-6 minutes on a high heat to reduce down and start making the chicken look darker and caramelized. Sprinkle with cilantro and serve on a bed of rice.