Cook maccaroni in boiling salted water about 10 minutes or until tender; drain. In saucepan, combine egg yolks, milk, butter, mustard, onion, salt and pepper. Add cheese. Cook and stir over medium heat about 10 minutes, or until mixture thickens slightly (do not boil). Stir in macaroni and bread crumbs. Fold in egg whites. Pour half the mixture into a 1 1/2 quart souffle dish. Place franks in casserole, reserving a few sliced franks for the top. Pour on remaining macaroni mixtue. Arrange reserved franks on top. Bake at 350 for 45 minutes. Garnish with parsley, if desired. Serve at once.