Melt the butter and add flour, blending well without browning. Add a half pint of water or Consomme, the juice of half of a lemon, and let it simmer twenty minutes. Season to taste. Take from the fire, add the yolks of two well-beaten eggs and the juice of an onion, and serve immediately. No Sauce into which the yolks of eggs are beaten must be set on the fire after adding these. To keep warm, set in a bowl of boiling water (in a bain-marie) till ready to serve. It is best served immediately.