In a medium saucepan cook macaroni according to package directions; drain and set aside.
Meanwhile, in a large saucepan combine the milk, cheese, and butter or margarine. Cook and stir over low heat until cheese is melted. Remove from heat. In a small bowl beat the egg yolks. Stir about 1/2 cup of the hot cheese mixture into the egg yolks. Pour egg mixture into saucepan, stirring to combine. Add the drained macaroni, bread crumbs, pimiento, parsley, and onion. Set aside.
Beat egg whites and cream of tartar until stiff peaks form (tips stand straight). Gently fold into macaroni mixture. Pour mixture into ungreased 1-1/2-quart souffle dish. Bake in a 325 degree F oven for 50 minutes. Serve immediately. Makes 6 servings.