Mix chestnuts, olives, sausage meat and mushrooms. Stuff Turkey with this mixture and sew closed. Place the turkey in a roasting pan with the breast up.
Lay pancetta strips across the breast. Season with salt and pepper and cover with olive oil. Roast in a slow oven, 300 F until tender (cook according to turkey directions).
When the turkey is nearly done, remove the pancetta slices, sprinkle the turkey with flour and baste it with pan juices. Return to the oven for 10 minutes. Repeat the basting several times while the turkey cooks for 30 more minutes.