Heat oven to 350 degrees. Arrange muffins in 13x9 inch baking dish. Beat egg whites in medium bowl until stiff, but not dry. Beat egg yolks with parsley and mustard in small bowl; fold into whites. Fold in cheese. Top each muffin half with egg mixture and 3 slices of the sausage. Bake 20 to 25 minutes or until egg mixture is set. Makes 6 open-face sandwiches.