Cut dozen hard boiled eggs in slices or quarters and place in saucepan. In another saucepan lightly brown a tablespoon butter, and one finely chopped onion. Sprinkle 2 tablespoons of flour in it and mix well. Add a pint of milk, stirring constantly, when well mixed, add juice of one lemon, a little salt and pepper; add one cup water a little at a time, stirring until the mixture becomes of uniform thickness, add three egg yolks and a heaping tablespoon of butter (more if you can spare it) but at no time allow the sauce to boil. Let it just reach the point of heat that will thicken it but not boil it. Pour over sliced eggs and let stand a few minutes to warm the eggs. Serve hot.