Melt the butter in a saucepan and add the onion, stirring with a wire whisk. Add the flour, stirring to blend. When blended, add the milk, stirring rapidly with the whisk. When blended and smooth, add the meat. Bring to the boil and remove from the heat.
Add the yolks, stirring rapidly with the whisk. Return to the heat and cook briefly. Add the nutmeg, salt (if t