In a bowl, toss the chicken pieces in 2 tbsp of cornflour until evenly coated. Heat 1 tbsp of vegetable oil in a large pan over a medium-high heat. Add the coated chicken and fry until golden brown and cooked through (about 5-6 minutes). Remove and set aside. In the same pan, heat the remaining 1 tbsp of vegetable oil. Add the chopped onion, red pepper, green pepper, and sliced carrot to the pan. Cook, stirring occasionally, for 5 minutes. Return the cooked chicken to the pan with the vegetables.