Wash the turkey, remove the giblets and dry. Torch any feathers, and wash and dry once more. Sprinkle with salt and pepper all over (inside and outside). Lightly simmer the giblets, drain, and chop.
Heat 1/2 cup butter and sautee the onion. Add the mince and chopped giblets. Add salt and pepper and continue sauteeing, stirring often.
Add the cognac or brandy and chicken stock, and simmer for 25-30 minutes until the liquid evaporates. Add the parsley and half the chestnuts (roughly chopped), and stuff the mixture into the bird.
Seal well using string, and place in an earthenware dish (you can also add potatoes around the bird).