Preheat the oven to 350 degrees.
Combine the shredded cheese with the mashed potatoes, sour cream and seasonings.
Beat the egg whites until stiff but still moist; set aside.
Beat the egg yolks until smooth and add them to the potato mixture.
Whip until light.
Fold in the beaten egg whites and pour into a buttered one and one half quart casserole.