In a large skillet cook the sausage meat, crumbled, over moderate heat, stirring and breaking up the lumps, until it is no longer pink, transfer it with a slotted spoon to a large bowl, and let it cool. Remove and discard the white tendons from the turkey fillets or chicken breasts with a sharp knife and in a food processor grind the fillets coarse. Add the sausage, grind the mixture until it is blended, and transfer it to the large bowl. Pour off the fat from the skillet, add the butter, and in it cook the onions and the celery with the sage, covered, over moderately low heat, stirring occasionally, until the vegetables are softened. Let the mixture cool in the skillet, add it to the large bowl with the ham, 2 cups of the bread cubes, crushed fine in a plastic bag, the remaining 1 cup bread cubes, left whole, the hazelnuts, the parsley, the eggs, the salt, and the pepper, and combine the mixture well. The stuffing may be made up to 1 day in advance and kept covered and chilled.