Sauce demi-glace is a rich, brown sauce commonly used in French cuisine. It is made by combining equal parts of brown stock and sauce espagnole, which is a basic brown sauce. The mixture is then simmered and reduced to concentrate its flavors, resulting in a thick, glossy sauce that enhances the taste of various dishes. This sauce serves as a foundation for many other sauces and can be used to complement meats, particularly beef and game. Its deep flavor profile makes it a favorite in fine dining, where it adds a touch of elegance to meals.