until it dissolves.
Meaning "half glaze" in French, classic demi-glace is a mixture of brown sauce and brown stock that has been concentrated. Because making demi-glace is a great deal of work, you'll probably want to buy it already prepared at gourmet shops or through mail-order catalogs. It's available in veal-, chicken-, duck-, fish-, and vegetable-flavored varieties. To use prepared demi-glace, you'll need to reconstitute it with water, using the proportions on the package. For best results, whisk the demi-glace into the water until it dissolves.