What is Demi-glace sauce?

Demi-glace sauce is a rich brown sauce often used by chefs. It's made from a reduction of clear stock and sauce espagnole - stock that has been thickened with a roux, diced vegetables and tomato purée - and is the basis for classic sauces such as Madeira, Diane and reform sauce. As an easier alternative for home cooking, you could use a good quality homemade beef stock that has been boiled and then reduced. Add a splash of Madeira or sherry for flavour.