Concentrated Brown or White Veal Stock

Often I don’t have room for one more container of stock in my freezer, so I make easy-to-store frozen concentrated stock cubes. As the stock boils, the water evaporates, concentrating the stock’s flavor and making what is called a demi-glace. (This technique of boiling to reduce the liquid is also a way to boost the flavor of an insipid stock—but only if there is no salt in it.) The wider the saucepan, the faster the liquid will evaporate, but it will still take at least 15 to 20 min