Demi-glace is a rich brown sauce of French origin which is used on its own and as a base for other sauces. It is one of the cornerstones of French cuisine, and although it is very time consuming to make, it has a distinctive flavor which some people feel is worth the effort. For people who want to enjoy demi-glace without hours of work, many markets sell it in prepared form. Prepared products vary in quality; as a general rule, the ingredient list should be filled with identifiable ingredients, not an assortment of mysterious chemicals.
This sauce combines equal parts veal stock with espagnole sauce, one of the “mother sauces” of French cuisine. The cook usually starts by preparing the veal stock, because it needs to simmer for a prolonged period, and then works on the espagnole sauce, which starts with a dark roux which is mixed with vegetables such as carrots, celery, and onions and simmered in stock with bones and pieces of meat. Both sauces require prolonged simmering before they can be drained and blended to make demi-glace, and the sauce requires even more cooking, as the sauce must be cooked until it has been reduced by half.
Some cooks use alternatives to veal stock in their demi-glace. If chicken stock is used, it becomes chicken demi-glace, while beef stock results in beef demi-glace, and so forth. If the sauce is simply referred to as “demi-glace,” it is assumed that it has been made with the traditional veal stock.
The original recipe for this sauce dates to Escoffier, a famous French chef. He may not have invented the recipe, but he certainly collected and codified it, along with numerous other French dishes. He also recommended an assortment of uses for the sauce, ranging from using it plain as a glaze on roasted meats to using it as the base for sauces, stocks, and soups. When well prepared, demi-glace has a very rich, intense, dark flavor which pairs well with a variety of meats and roasted vegetables.
Some people refer to demi-glace as “demi-glaze,” due to some confusion about the original French spelling. While this is not correct, it isn't far from the truth, as “glace” means “icing” or “glaze,” referring to the fact that the sauce behaves in a way similar to an icing. This famous sauce may also be called “brown sauce,” which is a bit of a misnomer, as several French sauces are brown, and a brown sauce does not necessarily have to involve demi-glace.