Preheat oven to 350ºF. Arrange muffin halves, split sides up, in single layer in 13"x9" baking dish; set aside.
Beat egg whites in medium bowl with electric mixer on high speed until stiff peaks form; set aside. Beat egg yolks, parsley and mustard in small bowl with wire whisk until well blended. Add to egg whites; stir gently until well blended. Gently stir in cheese; spoon evenly onto muffin halves. Top each with 3 sausage slices.
Bake 20 minutes, or until egg mixture is set.