Preheat the oven to 190C. Put the breasts on a chopping board, rounded side up. Use a small sharp knife to cut a pocket along the length of the breast, cutting as deep as you can without cutting trough to the other side. To make the stuffing, heat the oil in a pan and add in the onion, celery and thyme and cook for 3- 5minutes. Stir in the apples and cook for a further 2 minutes until it is soft but not falling apart. Stir in the cheese, Parsley and the prunes. Divide the stuffing between the breast pockets. Wrap them with Parma ham and rub the tops with a little oil. Transfer to the prepared tin and roast in a preheated oven for 20-25 minutes or until the chicken is cooked trough. Remove from the oven and cover in Foil. leave to stand for 3-5 minutes before serving.