Combine soy sauce, garlic and ginger in a large bowl. Add chicken and turn to coat. If possible, stand for 10 minutes to marinate briefly. Add tomato sauce, vinegar and sugar to the slow cooker and stir to combine.
Heat oil in a large frying pan over high heat. In 4 batches, cook the marinated chicken, turning, for about 2 minutes or until browned. Transfer to the slow cooker. Reduce heat to medium and stir-fry green shallot in the pan for 1 minute or until softened slightly. Add to the slow cooker.
Add red capsicum, green capsicum and pineapple to the slow cooker and gently stir to combine. Cover and cook on Low for 3 hours or until chicken is cooked through.
Place cornflour into a small bowl. Add ¼ cup of the liquid from the slow cooker and stir until smooth. Pour mixture into the slow cooker and stir to combine. Cover and cook for 30 minutes or until thickened slightly.
Serve sprinkled with reserved green shallot and sesame seeds.
