Ingredients: Butter, flour, eggs, cream, nutmeg, white stock.
Make a light-coloured roux by frying two ounces of butter and two ounces of flour, stir in some white stock and keep it very smooth. Let it boil, and add the yolks of three eggs, mixed with two tablespoonsful of cream and a pinch of nutmeg. Pass it through a sieve and use for masking cutlets, fish, &c.
See also: Wikipedia Cookbook.
Source: ''The Cook's Decameron: A Study In Taste, Containing Over Two Hundred Recipes For Italian Dishes'' from a project that puts out-of-copyright texts into the public domain. This is from a *very* old source, and reflects the cooking at the turn of the last century. Update as necessary.