In a large skillet, saute onion in butter until tender. Stir in the milk, mustard, salt and pepper; bring to a boil. Remove from the heat; stir in potatoes. Stir in egg yolks until blended. Cool at room temperature for 15 minutes. Stir in the cheese if desired.
In a small bowl, beat egg whites until soft peaks form. Fold into potato mixture. Spoon into a greased 1-qt. baking dish.
Bake at 350° for 40-45 minutes or until puffed and lightly browned. Yield: 6 servings.