Cook potatoes with salt in a covered saucepan in a small amount of water until tender. Drain. Saute onions in butter. Mash the potatoes. Beat onions, milk, salt and pepper into potatoes then add egg yolks. Beat well. Beat egg whites until stiff then fold into potato mixture.
Pile the potato mixture into a buttered 6 cup casserole dish. Bake at 375 degrees for 30 minutes or until lightly browned.