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. Cook and stir over low heat until cheese is melted. Remove from heat.
In a small mixing bowl, beat the egg yolks with a rotary beater. Stir about 1/2 cup of the hot cheese mixture into the egg yolks. Pour egg yolk mixture into remaining cheese mixture in saucepan, stirring to combine. Add the drained macaroni, bread crumbs, pimiento, parsley, and onion. Set aside.
In a medium mixing bowl, beat egg whites and cream of tartar with an electric mixer on medium to high speed until stiff peaks form. Gently fold beaten egg whites into macaroni mixture. Pour mixture into six ungreased 1-cup souffle dishes or an ungreased 2-quart souffle dish.
Bake in a preheated 325-degree oven for 25 to 30 minutes for 1-cup souffle dishes or about 45 minutes for 2-qt. souffle dish, or until puffed and browned. Serve immediately.