Demi-Glace

Step 1
Preheat the oven to 450 degrees F (230 degrees C). Place veal bones in a roasting pan.

Step 2
Roast bones in the preheated oven until well-browned, about 1 hour 15 minutes.

Step 3
While the bones are roasting, drizzle oil onto a baking sheet. Add onions, carrots, and celery, then spread tomato paste over top. Mix until vegetables are well coated.

Step 4
Roast vegetables on another rack in the oven until well-browned, about 45 minutes.

Step 5
Transfer vegetables and bones to a large stockpot. Pour 10 quarts water into the stockpot and set over medium-high heat.

Step 6
Meanwhile, pour 2 cups water into the roasting pan used for the bones. Set the pan on the stovetop over high heat; bring liquid to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon. Pour liquid and browned bits into the stockpot.

Step 7
Bring contents of the stockpot to a boil, then reduce the heat to low and simmer gently, skimming foam as necessary, until meat and all connective tissues are completely off the bones, about 18 hours.

Step 8
Set a colander over a bowl. Ladle vegetables, bones, and meat into the colander. Discard vegetables, bones, and meat; return any broth to the stockpot.

Step 9
Bring broth to a boil. Boil until broth reduces to approximately 1 gallon in volume, 30 minutes to 1 hour.

Step 10
Strain broth through a fine-mesh strainer into a large container set in an ice bath; chill to room temperature, 30 to 45 minutes. Cover the container with a lid or plastic wrap, then chill in the refrigerator until demi-glace is cold and set, 8 hours to overnight.

Step 11
Scrape and discard any fat from the surface. Turn demi-glace out onto a work surface. Cut into 16 blocks. Wrap each block in plastic wrap and store in a resealable plastic bag in the freezer.