Demi-glace - rich concentrated brown sauce

Demi-glace is a rich brown sauce of classic French cuisine. It is not one of the five mother sauces but a derivative of them. It is made by slowly reducing a mixture of espagnole sauce and brown veal stock (often roughly 1:1) by half, sometimes finished with Madeira or sherry. The result is a dark, glossy sauce with a velvety consistency that serves as the base for many further derived sauces. Preparation takes several hours.