This type of dish, more typical of the middle class than the nobility, first appeared in the eighteenth century, along with the thickening of sauces with flour and cream. In the nineteenth century, sauces were thickened with cream and an egg, and at the end of the cooking, separately sautéed onions and mushrooms were added. We prefer thickening with cream alone, as this is lighter. And we add a touch of vanilla. Untraceable by taste, it makes the sauce even more sensuous.