Mix one tablespoon of cornflour with 30ml of pineapple juice until smooth.
Combine the remaining juice with garlic, ginger, soy sauce, vinegar, sugar, ketchup, and chilli flakes.
Peel, core, and slice the pineapple and cut chicken into even pieces.
Stir-fry onion and peppers in oil for 3 minutes over high heat.
Coat chicken in cornflour, add to the pan, and stir-fry for 4 minutes.
Add pineapple and sauce, then simmer for 5 minutes until chicken is cooked.
Stir in the cornflour slurry and cook for 60 seconds until the sauce thickens.
Garnish with spring onions and serve hot.