Place the bones on a rimmed baking sheet and rub them with 1 tablespoon oil. Roast them, flipping them halfway through, until they are deeply browned, 35 to 45 minutes.
Meanwhile, in a medium bowl combine the carrots, onions, celery, garlic, tomato paste and remaining tablespoon of oil. Toss well to coat the vegetables in the tomato paste.
Remove the roasted bones to a large stock pot. Spread the coated vegetables on the hot baking sheet and roast, tossing halfway through, until evenly browned, about 20 minutes.
Add the vegetables to the stock pot along with the thyme, parsley stems, bay leaves and peppercorns.
Place the rimmed baking sheet over medium heat and add enough water to cover the bottom of the pan. Carefully use a wooden spoon to scrape up the browned bits from the bottom of the pan. Add everything to the stockpot.
Fill the stock pot with enough cold water to cover the bones by a few inches, about 8 quarts. Bring to a simmer over medium heat skimming any impurities that may rise to the surface as the mixture begins to warm. Reduce the heat to low to maintain a very gentle simmer. Simmer the mixture, skimming the fat as needed, until the stock very flavorful and reduced by a half, anywhere from 10 to 24 hours.
Strain the stock and cool rapidly in an ice bath. The stock is now ready to use or can be stored in the refrigerator for up to 5 days.