Preheat oven 375 degrees...Add milk to prepared mashed potatoes,cheeses,scallion,salt and pepper and egg yolks. Mix together throughly.In a small bowl with mixer on high speed,whip the egg whites until stiff peaks form.
Gently fold in egg whites into mashed potato mixture. Spoon the mixture into a greased 3 inch deep casserole dish. Bake about 50 minutes,or until puffed and golden. Serves about 5
Use mashed potatoes that are already prepared. Your favorite brand.