Preheat oven to 375°. Add milk to the prepared potatoes, cheeses, scallions, onions, salt and pepper and egg yolks. Mix together thoroughly until well blended. In a small bowl with mixer on high speed, whip the egg whites until stiff peak form. Gently fold beaten egg whites into mashed potato mixture. Spoon the mixture into a greased 3" deep casserole dish. Bake about 50 minutes, or until puffed and golden.