Healthy sweet and sour chicken

Name: Healthy sweet and sour chicken
Ingredients: 1 x 425g/15oz can pineapple chunks in natural juice; 2 tbsp cornflour; 2 tbsp dark soy sauce; 2 tbsp white wine vinegar; 2 tbsp soft light brown sugar; 2 tbsp tomato ketchup; ½ tsp dried chilli flakes; 2 boneless, skinless chicken breasts; 2 tbsp sunflower oil; 1 medium onion, cut into 12 wedges; 2 peppers, red, green, orange or yellow, deseeded and cut into chunks of about 3cm/1¼in; 1 x 225g/8oz can water chestnuts; 2 garlic cloves, peeled and crushed; 25g/1oz piece fresh root ginger, peeled and finely grated; freshly ground black pepper