Preheat the grill. Break the wing, hip and drumstick joints of the chicken so that it will remain flat during cooking. Place the chicken skin side up in a baking tray, brush with 15g/1/2oz of the butter and season with salt and pepper.
Grill for about 20 minutes until lightly browned. Turn and brush the cavity side with 15g/1/2oz butter and season. Grill for a further 15–20 minutes until tender.
Peel, core and chop the apples. In a small heavy saucepan, cook the apples, onion and celery in the remaining butter until tender. Add the croûtons and 75ml/3fl oz water, and season with salt and pepper.
Put the apple mixture into the chicken cavity, and grill the chicken for a further 5 minutes. Serve hot.