Preheat oven to 375°F.
Place the beef bones in a roasting pan or Dutch oven, not overlapping, and roast for about an hour, turning occasionally to brown and caramelize them.
2 pounds beef bones
Remove the bones from the oven and place in a stock pot. Remove and reserve the fat from the roasting pan.
Add 1/2 cup of water to the roasting pan and deglaze, scrapping up any cooked on bits with a wood spoon. Transfer to the stock pot along with the remaining 6 cups of water.
6 1/2 cups water
Bring to a boil, reduce heat and simmer.
Meanwhile, add 1-2 tablespoons of the reserved fat back to the roasting pan or Dutch oven over medium heat.
Add the onions, carrots and celery and cook until they start to brown.
1/2 onion, 1 celery stalk, 1 carrot
Stir in the tomato paste and continue to cook until it becomes a rich brown.
4 teaspoons tomato paste
Add the vegetables to the stock pot.
Place the bay leaf, garlic, peppercorns, parsley and thyme onto a small piece of cheesecloth and tie closed with a piece of twine. Add to the stock pot.
1 bay leaf, 3 peppercorns, 1 clove garlic, 1 sprig parsley, pinch dried thyme
Continue to simmer the stock for 6-8 hours, skimming any scum off that forms on the surface off periodically.
Strain through a fine mesh sieve and refrigerate to cool.
Skim any solidified fat off of the top of the stock once it has cooled.
Use or store, as desired.