Sauce Demi-Glace is a rich, brown sauce commonly used in French cuisine. It is made by combining equal parts of Espagnole sauce and beef stock, which are then reduced to intensify their flavors. This sauce serves as a base for many other sauces and is often used to enhance the taste of meats. To prepare Demi-Glace, the mixture is simmered slowly until it thickens and develops a deep, complex flavor. It can be served on its own or used as a foundation for sauces like Sauce Bordelaise or Sauce Chasseur. Its versatility makes it a staple in gourmet cooking.