To make the sauce, put the cornflour in a small bowl and stir in two tablespoons of the pineapple juice until smooth. Put the remaining pineapple juice in a separate bowl and stir in the garlic, ginger, soy, vinegar, sugar, ketchup and chilli flakes until thoroughly combined. Set aside.
Put the pineapple on a board and cut off the skin. Quarter lengthways and remove the tough central core. Cut the pineapple into thick slices and set aside. Cut each chicken breast into 7-9 even pieces (depending on size).
Heat the oil in a large non-stick frying pan or wok and stir-fry the onion and peppers for three minutes over a high heat. Coat the chicken in the cornflour and add to the pan. Stir-fry for four minutes until very lightly coloured on all sides.
Add the pineapple and sweet and sour sauce to the pan with the chicken and bring to a simmer over a medium heat. Cook for 4-5 minutes, stirring regularly until the chicken is cooked through and the pineapple is hot.
Stir in the cornflour and pineapple solution mixture and cook for 30-60 seconds until the sauce is thickened and glossy, turning the chicken and vegetables until nicely coated. Sprinkle with sliced spring onions and serve hot.