Demi glace, or half-glaze, is a classic French sauce that is used to garnish meat and which is also used as a base for some stocks and soups. A true demi glace is a mixture of half veal stock and half of a sauce called sauce espagnole. Sauce espagnole is made with carrots, onion, butter, celery, tomatoes and flour. Demi glace can be a complicated sauce to make, however, home cooks can find substitutes for the sauce.
Beef Bones Substitution
If you wish to tackle an authentic demi glace but are opposed to using veal bones to make the stock for ethical reasons, you can substitute beef marrow bones in the stock making part of the process. Many professional chefs are opposed to this substitution because they feel taking away veal bones removes the rich, creamy veal flavor, but you will be left with a sauce full of beef flavor.
Since a true demi glace is a time consuming, three-day process, a home cook can buy condensed stocks as a substitute. These are beef based stocks that come in a paste form. These stocks can be added to water, or even red wine, to create a passable substitute for demi glace.
Store-bought beef gravy can also be substituted for demi glace. The powdered packets that you add to water are inferior to jarred beef gravy. Jarred beef gravy will have some of the attributes of an authentic demi glace, including the right texture, thickness and color.
References and ResourcesI Hate Cooking Recipes: Making a Demi Glace
Eating Well: Beef Stock
Soup Basics: A Question of Stock