"This is my technique for veal demi-glace, and there's not much to it. I'm going for a pure veal stock reduction, fortified with nothing more than mirepoix and tomato. I don't do the classic roux-based espagnole sauce, which is traditionally mixed with veal stock and reduced by half."
Preheat oven to 450 degrees F (230 degrees C). Place veal bones in a roasting pan.
Roast bones in the preheated oven until well-browned, about 75 minutes.
While bones are roasting, drizzle oil onto a baking sheet. Spread onion, carrots, and celery onto baking sheet; spread tomato paste over the vegetables and mix to coat.
Roast vegetable mixture in the preheated oven until vegetables are well-browned, about 45 minutes.
Transfer vegetable mixture and bones to a large stockpot. Pour 10 quarts water into stockpot.
Pour 2 cups water into the bones roasting pan, set the pan over high heat, and bring to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon. Pour liquid into the stockpot with the bones.
Bring bones, vegetables, and water in the stockpot to a boil, reduce heat to low, and simmer gently, skimming foam as necessary, until meat and all connective tissue is completely off the bones, about 18 hours.
Set a colander over a bowl and ladle vegetables, bones, and meat into colander. Discard vegetables, bones, and meat; return any broth to the stockpot.
Bring broth to a boil; cook at a boil until liquid reduces to approximately 1 gallon in volume, 30 minutes to 1 hour.
Strain broth through a fine-mesh strainer into a large container set in an ice bath; chill to room temperature. Cover the container with a lid or plastic wrap and chill in the refrigerator until demi-glace is cold and set, 8 hours to overnight.
Scrape and discard any fat from the surface of set demi-glace. Turn demi-glace out onto a work surface. Cut into 16 blocks, wrap each block in plastic wrap, and place wrapped blocks in a resealable plastic bag. Store in the freezer.