Separate the whites and yolks of the eggs. Put the butter into a saucepan, add the onions, cut into very thin slices; shake until the onions are soft, but not brown, then dust over the flour, mix, and add the milk, salt, pepper and nutmeg. Stir carefully until this reaches boiling point, then stand it on the back part of the stove where it will keep hot for at least ten minutes. Beat the yolks of the eggs until very creamy, then stir them into the sauce, take from the fire, and fold in the well-beaten whites of the eggs. Turn into a baking dish or casserole and bake in a hot oven fifteen minutes; serve at once.