With a vegetable peeler cut a thin slice down the length of the rounded side of each celery rib so that the celery ribs can lie open side up. In a bowl combine well the lemon juice, the oil, and the apple, stir in the Roquefort,the walnuts, the parsley, and salt and pepper to taste, and combine the mixture well. Divide the mixture among the celery ribs, pressing it into place, and cut each rib crosswise at an angle into 1-inch slices. The hors d'oeuvres may be made 3 hours in advance and kept covered and chilled.
