Heat your oven to 375F.
Spread the bones in an even layer on a sturdy baking sheet and roast for 90 minutes. Make sure your baking sheet has sides at least 3/4" high as the bones will throw a lot of fat.
Remove fat and sprinkle the carrots, onions, shallots and leeks overtop. Roast another 30 minutes or so.
Place bones and aromatics in a large stock pot, add the tomato paste and thyme and simmer gently (I mean gently) for around 18-24 hours.
Remove from heat. Let cool slightly, remove bones and strain through a fine strainer.
Let cool completely. Skim any surface fat.
Return to a pot and simmer gently to a final volume of 2-3 cups.
Portion demi-glace in an ice cube tray and freeze. Place cubes in a zip loc bag or better yet vacuum pack 3 or 4 cubes per package.
Use on your unsuspecting guests to amaze them.